﻿July, 1896.] 



CHARACTERS OF ENAMEL PATTERN. 



25 



Blasius based Lis classification on the pattern of enamel folding witli- 

 ont regard to external cliaracters, and Lataste snbdivided tlie group 

 according to tlie numbers of mammae and plantar tubercles, disregard- 

 ing everytliing else. The impossibility of reaching satisfactory results 

 by any of these methods has been pointed out by Biichuer, who, how- 

 ever, takes an equally extreme position in his reluctance in any way to 

 subdivide the genus Microtus. 



Biichuer was first to recognize the important fact that the enamel 

 pattern, while variable within certain limits and hence of little value 

 taken by itself, is nevertheless of considerable systematic importance 

 when considered in connection with other characters.^ 



In about 75 per cent of the specimens of a given species the enamel 

 pattern conforms to a type which maybe considered normal.^ Among 

 the abnormal sj^ecimens constituting the remainder, the variation, how- 

 ever, is very considerable. In the accompanying illustrations (figs. 1, 

 2, 3, 4, 5, and 6) are shown some of the conspicuous aberrations in the 

 form of the teeth of Microtus ])ennsylimnicus.^ In the descriptions which 

 follow the normal enamel pattern is alone considered. 



1 After meutioning Lataste's view (see footnote, p. 23), Biichuer says: ''Meiner 

 Ansiclit nach liefert Ira Gegentbeil der Ban cler Backenziihne, obwohl derselbe 

 zii weilen aucli im Bereiche einer Art leicbt variirt, ein vorziiglicbes Merkmal, welcbes 

 alleiii genommeii fiir die Cbarakteristik einer Art nicbt geniigt, in Yerbindung aber 

 mit den iibrigen Merkmalen sebr grosse Dieuste leistet und von bedentendem sys- 

 tematiscben Wertbe ist,'' (Wissenscbaftlicbe Resultate der von N. M. Przewalski 

 nacb Ceutral-Asien nuternommenen Reisen. Zool. Tbeil, Bd. I, Siiugetbiere, Lief. 3, 

 1889, p. 97.) 



'-'Among 285 specimens of Microtus pennsylvanicus 71, or 21.9 per cent bave tbe 

 enamel pattern in some way abnormal. Of tbese, 26, or 9.1 per cent, bave tbe first 

 outer triangle in m 3 communicating more or less freely witb the inner triangle 

 (fig. 3); one bas tbe second outer triangle opening into tbe posterior loop (fig. 3) ; 

 two bave tbe posterior loop of very unusual sbape (fig. 3) ; one bas a second inner 

 closed triangle in ra 3 (fig. 3), and 14, or 4.9 per cent, show a distinct fourtb salient 

 angle on tbe outer side of tbe same tootb. In tbe first lower molar 24, or 8.3 per 

 cent, liave 6 closed triangles (fig. 4), one has only 3, still another has 7 (fig.4), 

 while in 5, or 1.7 per cent, there are 4 (fig. 4). Of tbese 285 specimens m 3 is abnormal 



in 44 cases, or 15.4 per cent, m 1 in 31 cases, or 10.8 per cent. Grouping the abnor- 

 malities according to their frequency, they may be arranged as follows: 

 m 3 bas first outer triangle open in 26 cases, or 9.1 per cent. 



m 1 bas one additional triangle in 24 cases, or 8.3 per cent. 



m 3 bas an additional salient angle on the outer side in 14 cases, or 4 per cent. 



m 1 bas one less triangle than usual in 5 cases, or 1.7 per cent. 



m 3 has tbe posterior loop of very unusual shape in 2 cases, or 0.7 per cent. 



m 3 has the second outer triangle abnormal in 1 case, or 0.35 per cent. 



ni^ bas an additional inner triangle in 1 case, or 0.35 per cent. 



m 1 has two additional closed triangles in 1 case, or 0.35 per cent. 



m 1 has two less closed triangles than usual in 1 case, or 0.35 per cent. 

 The drawings here reproduced are all from specimens taken in the eastern and 

 central parts of the United States and adjoining British Provinces. They are 

 selected from the series of about 170 belonging to the United States Department of 

 Agriculture. 



