22 



bulletin: museum of comparative zoology. 



specimen was acquired by the Museum of Comparative Zoology at 

 Cambridge, the other by the American Museum of Natural History, 

 New Yorli. One will readily appreciate the importance of this example 

 when it is remembered that but one other Mylostomid is known in 

 which nearly the complete dentition occurs in natural association with 

 portions of the headshield and abdominal armor. The second specimen 

 referred to will be found hereinafter described as the type of a distinct 

 genus, Dinomylostoma. 



The structural characters of Mylostoma have been worked out with 

 such thoroughness and precision by Dean in his elaborate monograph 

 of 1901, that it would be futile to attempt to supplement hia descrip- 

 tions. Only in a few pai'ticulars is a somewhat different reading of tlie 

 original to be advocated than that which is preferred by him. Thus, 

 the construction which we should place upon the dental elements of 

 the upper jaw is indicated in Fig. E, the evidence for which rests 



Fig. E. — Upper dentition of Mylostoma variabile Newb., from the Cleveland shale of 

 Sheffield, Ohio, x i. 



upon the following considerations : — (1) In no other position is there 

 such accurate fit between upper and lower dental plates when the jaws 

 are closed ; (2) it is the only arrangement which accounts at all points 



