EASTMAN: THE DINICHTHYIDS. oo 
Descrirtions or New SPECIES. 
Under this heading are included, 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 characters 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 Dintehthys, 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 Coccosteide, 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 different degree, 
and is terminated in a different manner, amongst the several genera. 
In Ooecosteus 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 
