| 494 The American Naturalist. [June, 
sullivanti, which was weathered in such a manner as to reveal 
the under surface of the head. Reference will hereafter be 
made to this description, which is in every way a notable one, 
and the cranium on which it was based. For an opportunity 
of studying the latter, together with other valuable specimens 
preserved in the museum of-Ohio State University, the writer 
is greatly indebted to Dr. Edward Orton. 
Besides these specimens, the writer has examined a large 
amount of material belonging to different public and private 
collections, and is thus enabled to supply certain deficiencies 
in our knowledge of the leading species M. sullivanti. Only a 
brief exposition of the cranial characters can be attempted 
within the limits of the present article, a more detailed discus- 
sion being reserved until another time. 
The cranium of Macropetalichthys is to be conceived as a 
comparatively thin, flexible box or capsule, capable of with- 
standing a good deal of distortion without rupture. It is com- 
posed of plates united by squamosal sutures, and traversed 
centrally by the sensory canal system. The posterior bound- 
ary of the cranium is deeply concave in the middle, and its 
postero-lateral angles are produced backwards for a consider- 
able distance, over-riding a structure called by Cope the 
“nuchal plate.” The elements taking part in these cranial 
prolongations are probably homologous with the epiotic and 
marginal plates of other Coccosteids; although no definite 
sutures have been observed between them, a marked depres- 
sion occurs, extending from the extremity of the posterior 
angle forwards toward the centre of the squamosal plate, 
and this depression may represent the natural boundaries of 
the epiotic and marginal plates. This depressed line corre- 
sponds with the externo-lateral suture of Newberry’s “ parietal? 
plate,” as represented in his diagram. The externo-lateral 
boundary of his so-called “squamosal ” is only the outer mart- 
gin of the cranial prolongations just described; their inner 
margins have not been previously shown, but are represented 
in Pl. XII, fig. 4. 
* Palaeozoic Fishes of North America, (Monograph U. S. Geol. Surv., Vol. XVI 
1889), p. 43. 
