A MONOGRAPH OF THE BATS OF NORTH AMERICA. 31 



heel or hypocoiiid is large. It is united to the triaugle by a conimis- 

 sure at the lingual side. Such a commissure is provided with a sharp 

 ciisp in P. perotls^ but as a rule it is smooth. 



The student can not fail to be imju-essed with the variable form of the 

 lateral incisor in both Jaws. In the lower jaw it may be of the same 

 character as the other teeth in its serfes, as in Adelonycteris, Vespcnujo, 

 and Las-ionycterLs. It may be more robust than its fellows, as in Ves- 

 2)ertilio, but oftener is simpler in outline, as in Corytiorhimts, and may 

 be not only so, but dwarfed in all proportions, as in Xyctinomiis ]>)-(tsU- 

 iensis. The next step may be anticipated, namely, its entire disap- 

 pearance, as in l^yctiitomHs niacrotis and in Promops. 



Tni'se nsodilications suggest that the tooth has a struggle to main- 

 tain its ovv-!i ill the dental armature. At all events its study is one of 

 interest, and should never be omitted in defining the characteristics of 

 all forms whatever, either of variations, species, or genera. 



The first and second jiremolars in both the upper and the lower jaw, 

 and the third incisor in the lower jaw, often present a "^)erfect cingulum. 

 This is distinctly seen in the first premolar of Atalapha. Such a tooth 

 is remarkable since, as far as I know, it is without parallel elsewhere in 

 mammalian teeth. Various degrees of development of the buccal 

 aspect of the cingulum appear to give rise to a cusp, which then arises 

 after the outer or buccal cusp. I was led, in an early study of the 

 human dentition, to claim that this the outer (labial) cusp of a bicuspi- 

 date form arose first and the inner (buccal) cusp arose Liter. I have 

 long maintained (Dental Cosmos Phila. 1874, G17, also Studies in the 

 Facial Region, 1874) the primitive tooth to have been monocuspidate 

 and the subsequent forms to have been developments therefrom. But 

 my claim has not been allowed. 



An excellent view for contrasting taxonomic characters is that ob- 

 tained by examining with a lens the lingual aspect of the lower mo- 

 lars. On the whole, the first molar yields the most trenchant char- 

 acters. Three cusps are seen in profile, named, from before backward, 

 the paraconid, metaconid, and hypoconid. The proportions of these 

 to one another should be made in all studies of the teeth. In ptero- 

 pines the three cusps are merged, and a simple convex contour results. 

 A similar effect is produced in Hemiderma and Brachyphylla among the 

 phyllostomines. For the most part the three cusps are separately dis- 

 played as acute triangles, Avhose bases are confiuent. In rhinolo- 

 phines the hypoconid is fiat, produced backward, as is also the case in 

 GhooeronyoteriH and PhyllonycieriH among the glossophagines, but in 

 other genera it is acutely cnsped and co-equal with the two preceding- 

 elements. In Natalus it is actually the largest of the series. 



