748 THE AMERICAN NATURALIST. (VoL. XXXII. 
prove this species to be the most primitive member of the 
genus known. It is now proposed to illustrate its osteology 
more fully, and at the same time offer comparisons with other 
Arthrodires, including Coccosteus and Titanichthys. The two 
last-named genera, in fact, may be taken to represent the 
extreme limits of the family Coccostetde. For in whatever 
grouping Dinichthys be placed, be it of subfamily rank or other- 
wise, there is no question that Titanichthys should accompany 
it; and the relations of Dinichthys to Coccosteus are seen to 
be so intimate, we are unable to remove it from the same 
family. On the other hand, the separation of Macropetalich- 
thys and some other Arthrodires from the Coceosteide@, where 
they are now commonly placed, seems advisable. 
Dinichthys pustulosus. 
Besides the examples of this species preserved in the Museum 
of Comparative Zoology, of which the most perfect cranium is 
from Rock Island, the writer has been able to consult a num- 
ber of fine specimens belonging to Messrs. Teller, Monroe, 
and Slocum, of Milwaukee, two from the Cedar Valley Lime- 
stone belonging to the State University of Iowa, and one 
belonging to the United States National Museum, of which last 
an illustration is given herewith (Fig. 1). The original of this 
was kindly loaned by Mr. F. A. Lucas, curator in charge of 
comparative anatomy; the Iowa material by Prof. Samuel 
Calvin ; and the Milwaukee specimens by their owners, to all 
of whom grateful acknowledgments are hereby rendered. 
The larger and more specialized species of Dinichthys and 
Titanichthys have a nearly flat cranium, and the surface of all 
the derm plates is smooth, these probably having been cov- 
ered in life by the integument. The cranium of D. pustulosus, 
on the other hand, is strongly arched from side to side, and, 
like all the body plates, is covered with innumerable small 
rounded tubercles, slightly stellate at the base. A narrow 
band along the sutures, however, is generally striated and 
destitute of tubercles; and the suture lines themselves are 
undulatory. In all of these particulars the species bears a 
