24 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



feature, and serves at the same time to emphasize the transitional nature 

 of this genus, consists in the beak-like termination of the mandibles in 

 front, together with their deeply concave and approximately even oral 

 margin. The general contour of the mandible, apart from lateral thicken- 

 ing of the oral margin, is Dinichthys-like ; but precisely this thickened 

 condition imparts to it a certain Palaeomylus-like aspect. As regards 

 functional adaptation, therefore, Dinichthys and Dinomylostoma are re- 

 lated to each other in much the same way as are Rhynchodus and Palaeo- 

 mylus amongst Chimaeroids. Approach to Dinichthyid conditions is to be 

 noted also in the slightly prehensile form of the vomerine teeth of 

 Dinomylostoma. These teeth, if found in the detached condition, might 

 readily be mistaken for the so-called " premaxillaries " of Dinichthys. 

 On the other hand, the palato-pterygoid dental plates are typically 

 Mylostomid. If they in their turn were known to us only in the 

 detached condition, they would be unhesitatingly assigned to the genus 

 Mylostoma. One perceives, accordingly, that the new form acquaints 

 us with an interesting intermediate stage of modification between two 

 well-marked types of Arthrodires, of which the Mylostomid is clearly 

 the more primitive. 



Our knowledge of the form sub judice is derived from a unique 

 example, the history of which is as follows : Collected in the year 1868 

 from a shaly outcrop of Portage beds upon the farm of John Pierce, 

 near Mt. Morris, New York, it was acquired by the late Prof. 0. C. 

 Marsh, and by him deposited in the Yale Museum. Here it remained 

 for many years stored away and apparently forgotten. The writer's 

 attention was first called to it by his lamented friend Professor Beecher, 

 shortly before the unfortunate loss to science of the latter. Through 

 the kindness of his successor, Prof. Charles Schuchert, the specimen was 

 committed to the present writer for further preparation, study, and 

 description, and for this privilege cordial thanks are here offered. At 

 Professor Schuchert's suggestion, also, the specific title is inscribed to 

 the memory of his distinguished friend and predecessor. 



The type specimen upon which the species is founded presents the 

 following parts for examination : nearly the complete dentition ; frag- 

 mentary portions of the headshield ; a part of the left suborbital shown 

 in Fig. F ; the dorso-median, substantially perfect ; and two of the 

 ventro-lateral plates, one complete, the other in impression. 



Naturally, chief interest is engaged by the dentition. Both man- 

 dibles are preserved, and are shown from the external and internal 

 aspects in Plates 4 and 5 respectively. The anterior margin rises into a 



