6. Stalk more or less straight, base not deflected. _Microtus oeconomus_, p. 204
6". Stalk spatulate, and base deflected from axis of shaft....
_Microtus guatemalensis_, p. 198
7. Base enlarged, depth nearly 1/2 of breadth.... _Lemmus trimucronatus_, p. 193
7". Base moderately enlarged, depth near 1/3 of breadth....
_Microtus pennsylvanicus_, p. 206, or _Microtus townsendii_, p. 204
8. Base hour-gla.s.s shaped as viewed from proximal end....
_Phenacomys intermedius_, p. 197
8". Not so 9
9. Lateral processes separated from tip of shaft by more than the thickness of the lateral process 10
9". Lateral processes separated from tip of shaft by less than the thickness of the lateral process 11
10. Lateral processes more than 1/2 the width of median process.... _Microtus longicaudus_, p. 201
10". Lateral processes slender, less than 1/2 the width of median process.... _Microtus monta.n.u.s_, p. 204
11. Lateral ossifications equal in length to median ossification.... _Clethrionomys_, p. 194
11". Lateral ossifications shorter than median ossification 12
12. Size small, less than 3.4 mm. in total length....
_Microtus oregoni_, p. 199
12". Size medium, more than 3.4 mm. in total length 13
13. Greatest width of stalk at a point about 1/3 of length of stalk from base.... _Microtus chrotorrhinus_ (Hamilton, 1946:382).
13". Greatest width of stalk at a point less than 1/3 of length of stalk from base.... _Lagurus curtatus_, p. 210
14. Size of baculum larger, base more than 3 mm. wide, processes all well developed.... _Ondatra zibethicus_, p. 198
14". Size of baculum smaller, base less than 3 mm. wide, processes poorly developed in some animals.... _Neofiber alleni_, p. 209
15. At least one digital ossification present 16
15". Digital ossifications not present.... _Dicrostonyx groenlandicus_, p. 193
16. Breadth of stalk at least 1/2 length of stalk 17
16". Breadth of stalk less than 1/2 length of stalk 19
17. Length of stalk greater than 3.6 mm. and less than 1-1/2 times its greatest breadth.... _Microtus richardsoni_, p. 199
17". Length of stalk usually less than 3.6 mm., or if more than 3.6 mm. (up to 4.0 mm.) then length 1-1/2 times or more its greatest breadth 18
18. Median process attenuate distally in dorsal view, and relatively long (more than twice its own breadth), 1/5 to 3/5 the length of stalk; breadth of stalk usually 2/3 or more length of stalk.... _Microtus miurus_, p. 200
18". Median process relatively blunt distally in dorsal view, relatively short (usually less than 1/4 length of stalk), breadth of stalk usually less than 2/3 length of stalk....
_Pitymys_, p. 208, _Pedomys_, p. 207, or _Microtus mexica.n.u.s_, p. 205
19. Distal processes small and firmly ankylosed to distal end of shaft.... _Phenacomys longicaudus_, p. 197
19". Distal processes if present not firmly ankylosed to distal end of shaft 20
20. Dorsal concavity of base as viewed from proximal end usually deeper than ventral concavity.... _Microtus mexica.n.u.s_, p. 205
20". Dorsal and ventral concavities of base equal in depth or ventral one the deeper 21
21. Total length of baculum more than 3.6 mm.... _Microtus californicus_, p. 205
21". Total length of baculum less than 3.6 mm.... _Synaptomys cooperi_, p. 194
DISCUSSION
Owing to shortness of lower incisors and present geographic distribution of the species, Hinton (1926:35) considered the Tribe Lemmi (lemmings) to be more primitive than the Tribe Microti (voles). The surviving lemmings are specialized in many features and therefore are considered as advanced end-products of an evolutionary radiation of a primitive microtine stock, of which all earlier stages are extinct.
Hinton regarded _Dicrostonyx_ as the most primitive of the genera of lemmings on account of its more complex molar teeth (complexity was considered to be primitive), and on account of the presence of three primitive longitudinal rows of tubercles in unworn molars. The other three genera were arranged in order of increasing specialization as follows: _Synaptomys_, _Myopus_, _Lemmus_.
If the baculum tended to retain its primitive character while specializations in the external anatomy developed, and if the above arrangement is correct the most primitive bacula would be found in _Dicrostonyx_ and in _Synaptomys_. The baculum in these two genera in comparison to that in _Myopus_ (as figured by Ognev, 1948:512) and _Lemmus_ has a slenderer stalk and smaller digital ossifications or none at all. The baculum in the genera of lemmings increases in robustness and the development of processes from _Dicrostonyx_, to _Synaptomys_, to _Myopus_, to _Lemmus_--the same order outlined above for total of specialization. The two extremes in this series are near the extremes of variation in bacula to be found in all microtines. The baculum in lemmings as a group cannot then be considered more primitive than in voles as a group, although the voles are usually considered to be more advanced. The situation in the voles, as we shall see, casts a different light on the matter.
The voles, Tribe Microti, were considered by Hinton (1926:40) to be more advanced than the lemmings because the incisors of the voles are longer and the root of their last lower molar is lingual to the root of the incisor. Hinton thought also that the murine ancestors of microtines had shorter incisors and that the backward extension of the incisors in the voles is a more ancient feature than the hypsodonty of the molars. A trend in the molar teeth has been toward greater hypsodonty. The voles in which the molars are least hypsodont are thus considered primitive. These include the living genera _Clethrionomys_, _Phenacomys_, _Ondatra_, _Dolomys_, _Ellobius_, and _Prometheomys_. Therefore, the baculum, in these a.s.sumedly primitive genera, would be expected to resemble the baculum in the lemmings or at least the most primitive lemmings. This is not the case.
The bacula that I have examined of _Clethrionomys_ and _Phenacomys_ have well-developed digital ossifications. In this they resemble the baculum of the genus _Lemmus_, the most advanced genus of lemmings according to Hinton. The baculum of _Dolomys_ has not been studied. The baculum in _Ondatra_, and in _Prometheomys_ as ill.u.s.trated by Ognev (1948:552), also possesses well-developed processes. The baculum of _Ellobius_ is small and lacks processes (as figured by Ognev, 1950:662). No ossification was found in a single specimen of _Ellobius_ examined by me although the entire glans p.e.n.i.s was removed and cleared without dissection. So far as known then, with the exception of _Ellobius_ and _Phenacomys longicaudus_ (Dearden, 1958:547), the primitive microtines having rooted molars possess bacula having three well-developed ossified processes.
Voles of the genus _Microtus_ vary in the structure of the baculum almost as much as do the lemmings. Within the single subgenus _Microtus_ some individuals of _Microtus mexica.n.u.s_, for example, have minute ossified lateral processes and other individuals lack these processes; _Microtus pennsylvanicus_ and some other species have proportionately large lateral ossifications. If the well-developed condition of the baculum in the microtines having rooted molars is primitive, then within the genus _Microtus_ those species having well-developed bacula may be considered primitive.
The genera _Lagurus_ and _Neofiber_ have moderately developed or well-developed lateral processes. _Neofiber_ exhibits a tendency, not prominent elsewhere, to have a proportionately smaller median process rather than reduced lateral processes.
American species of _Microtus_ (genus and subgenus) that have moderately- to well-developed ossified lateral processes are _M. townsendii_, _M.
oeconomus_, _M. pennsylvanicus_, _M. monta.n.u.s_, and _M. chrotorrhinus_.
_Microtus_ of other subgenera having this type of baculum include _M.
(Herpetomys) guatemalensis_, _M. (Chilotus) oregoni_, and _M. (Chionomys) longicaudus_.
American species of _Microtus_ (genus and subgenus) in which the lateral ossifications are weakly developed or absent (although cartilaginous lateral processes are present) include _M. mexica.n.u.s_ and _M.
californicus_. In other subgenera, species of _Microtus_ having reduced lateral ossifications are _M. (Pedomys) ochrogaster_, _M. (Pitymys) pinetorum_, _M. (Pitymys) parvulus_, _M. (Pitymys) quasiater_, _M.
(Arvicola) richardsoni_, and _M. (Stenocranius) miurus_.
The microtines are essentially holarctic in distribution. Both of the tribes, the lemmings and the voles, as well as primitive representatives of each tribe (not considering _Ellobius_) occur in both the Old World and New World. It is not certain on which continent (or continents) the Microtinae first differentiated. It is certain, however, that at various times, both early and late in the evolution of the subfamily, representatives have crossed from Eurasia to North America or _vice versa_. Each of 10 or more microtines in the New World is more closely related to some microtine in the Old World than to any other microtine in the New World.