1880.] NATURAL SCIENCES OF PHILADELPHIA, 227 



being valuable guides in interpreting the plan of the molars. He 

 traced the shapes of the last-named teeth from the sub-conical form 

 of the canine, with its associated cusplets or cingules characterizing 

 the canines, up to the complicated figures of the molars. Among 

 the seals, Leptonyx exhibits to the best advantage the figure result- 

 ing from the pronounced development of the antero-posterior cus- 

 plets, and is of still greater interest inasmuch as the molars retain 

 in all essential features the same parts. In genera where the form 

 of the molars is not so retained, the manner after which the depar- 

 ture takes place in the upper jaw is as follows : 



1. The buccal cingulum becomes developed. 



2. The buccal surface of the main' cusp is directed obliquelj^ 

 backward and inward, and at the same time becomes concave. 



3. In genera having the W-shaped pattern, the first V answers to 

 the concave, obliquel}^ placed buccal surface of the main cusp. 

 The second V is a vegetative repetition of the first, and is formed 

 from the posterior cusplet of the canine. 



The W thus formed is a conspicuous feature in the molars of 

 most Insectivora and Chiroptera. It can be traced through its 

 several stages of development from the Carnivora. The genera of 

 the Procyonidjne exhibit the transition adYantageously. Tiie W 

 of the upper jaw, while forming a portion of the free under-surface 

 of the crown, is not functionally active as part of the grinder, but 

 is an extremeJij ohiiquehj placed portion of the i^hearing buccal 

 surface, and is not articular. 



The V V seen from the palatal side of tooth form the summits 

 of two downward-projecting, prismoidal, shearing columns. Ex- 

 amined in relief from before backwards these columns are seen 

 to be of different relative lengths. In Vespertilio and Antrozous, 

 for example, where the appearance of the under free surfaces of 

 the crowns are almost identical, conspicuous differences in the 

 lengths of the columns are detected when the teeth are examined 

 with the columns in antero-posterior relief. 



Tiie elevations placed to the palatal side of the base of the 

 columns are developments from the palatal fold of the cingulum 

 of the caniniforni tooth. If oue cingule be alone developed it lies 

 to the median side of the first V. Should a second be present, it 

 lies in an analogous position to the second V, and is much less 

 pronounced than the first. 



The differences in the forms of the lower molars are traceable 

 to similar modifications of tlie simple cone and associated cusplets. 

 The second V is incomplete, the anterior limb not joining the 

 first to form a true W. There is no disposition to form a lingual 

 outgrowth. In its stead a tendency to backward projection from 

 the base of the second V exists. This projection is conveniently 

 called the " heel " of the tooth, and is always articular. 



The forms of the canines and premolars are not as simple and 

 imiform as they- at first sight appear. They often present remark- 

 able differences in their details. This is especially true of these 



