EASTMAN: THE DIXICHTHYIDS. 33 



Descriptions of New Species. 



Under this heading are induded, besides species altogether new to 

 science, certain others which are now demonstrated for the first time to 

 belong to the genus Dinichthys. The subject may be properly intro- 

 duced by a consideration of the latter forms first. 



As is well known, a large number of genera and species of Arthrodires 

 have been founded on detached fragments, which commonly yield but 

 little insight into the structure of the fish as a whole. Sometimes our 

 knowledge of these forms is increased by the discovery of more perfect 

 specimens, or by finding parts in natural association with the dentition 

 or with other parts. The dentition obviously yields the most trenchant 

 characters that can be employed for the discrimination of species ; but 

 in Dinichthys scarcely less important charactei's are furnished by the 

 dorso-median plate. Owing to the massiveness of this plate, it is not 

 readily subject to fracture or distortion, and is perhaps of more frequent 

 occurrence than any other plate in the body. Its configuration varies 

 markedly amongst the different species of Dinichthys, but remains 

 fairly uniform within the limits of one and the same species ; hence its 

 systematic importance is very great. 



There is one feature about the dorso-median which appears to be 

 peculiar to the Dinichthyids ; or, to put it differently, the Dinichthyids 

 are distinguished from remaining Coccosteids by the possession of a cer- 

 tain characteristic structure ; and this is the large, excavated carinal 

 process by which the dorsal shield is terminated posteriorly. (See 

 Plate 2, Figs. 3, 4 ; Plate 3, Figs. 2, 3.) All of the Coccosteidce, so 

 far as known, have a median longitudinal keel or ridge on the inferior 

 surface of the dorso-median ; but it is developed to a diff"erent degree, 

 and is terminated in a different manner, amongst the several genera. 

 In Goccosteus it ends posteriorly in a simple blunt spine ; in Homosteus 

 the ridge is stronger, and terminates in a knob at the posterior border 

 of the shield ; and in Heterosteus the keel is greatly developed, but is 

 not produced behind the margin to any great extent, nor is it excavated 

 superiorly. This series of Coccosteid genera leads up to the conditions 

 that exist in the Dinichthyid group, where the inferior ridge is termi- 

 nated posteriorly by a distinct process, such as is unknown in other 

 members of the family. If we arrange the Dinichthyid forms in order 

 of relative development of the carinal process, we shall have the follow- 

 ing series: Coccosteus sp. Pander (hereinafter described as D. livoni- 



VOL. XXXI. — NO. 2. 3 



