498 The American Naturalist. [June, 
frontals, as these are recognized in Dinichthys, and the infer- 
ence is that fusion has taken place more or less extensively 
among the remaining plates. That the median series should 
consist of but two plates, central and pineal, appears quite 
remarkable. As names have already been applied to the 
system of paired plates by Cope, it appears advisable to retain 
them, not because all of them express undoubted homologies, 
but merely for convenience of distinction. If this author 
has delineated the boundaries of the maxillary plate correctly 
in M. rapheidolabis, we have here a marked difference between 
the type-species and M. sullivanti. 
A small pineal foramen occurs at a point about half-way 
between the frontal lyra formed by the sensory canals and the 
extremity of the muzzle. Its position is indicated in the cra- 
nium figured by Newberry,’ although no significance was at- 
tributed to the structure there shown. The opening appears 
to be capped by a small operculum as in Titanichthys. Traces 
of a pineal foramen are also observable on a weathered speci- 
men for which von Koenen’? has established a new genus, Holo- 
petalichthys ; but as the configuration of the cranium agrees 
essentially with that of Macropetalichthys (cf. fig. 4), and is from 
an equivalent horizon, we venture to regard H. novaki as per- 
taining to the latter genus. 
A description of the osteology in detail, including the in- 
ferior structure of the cranium, is beyond the scope of the 
present paper. It is hoped, however, that the diagrams given 
herewith will suffice to show the general arrangement of the 
cranial bones as they appear on the dorsal surface. The 
writer desires in conclusion to acknowledge his especial in- 
debteduess for the loan of specimens to the following persons: 
Dr. Edward Orton, Director of the Ohio Geological Survey ; 
Prof. A. A. Wright, of Oberlin College ; Prof. R. P. Whitfield, 
of the American Museum of Natural History ; Prof. J. F. Kemp, 
of the Columbia School of Mines; Prof. C. H. Hitchcock, of 
Dartmouth College; and Mr. F. K. Mixer, Curator of the Buf- 
falo Society of Natural Sciences. 
cae ice of ee Zoology, Cambridge, Mass. 
of a America (Monograph U. S. Geol. Surv., Vol. 
XVI D 180) Pi evii 
i enid Gecsllech. Wissen., Vol. XL, (1895), pl. iv, fig. 2. 
