FOSSIL FISHES. 33 
others being distorted and narrowed, but the originals were all from the 
same place, Turner’s Falls, where this is the most abundant species, and 
they show the peculiar erect, thickened dorsal scales, which are not devel- 
oped to the same degree in any other. Unfortunately all the figures of 
I. tenuiceps yet published are taken from imperfect specimens. That on 
Pl. IX of the American Geology, pt. 6, represents the posterior half of the 
body fairly well, but the head is a shapeless mass, and the arch of the back 
is only partially shown. 
As mentioned in the remarks on the genus Ischypterus, the preserva- 
tion of the head is so generally incomplete that we must conclude its bony 
structure was delicate and largely reénforced by cartilage. Out of the large 
number of specimens which I have, however, a few give the outlines of the 
head and much of its structure with considerable accuracy. From these 
we learn that it was conical, rapidly sloping from the high nuchal arch, and 
from the smallness of the mouth, pointed at the muzzle. The general form 
was ovate, in that respect resembling Ischypterus ovatus W. C. R., but the 
species may be distinguished generally at a glance by the high, thickened, 
and often obtuse scales which crown the humped back. The length of this 
species in mature individuals is eight inches and the breadth immediately 
behind the head is two and a half to three inches. The dorsal scales are 
often strangely thickened and distorted in the nuchal region, where they 
are sometimes more than half an inch long, clavate and blunt. This I was 
at first disposed to regard as the result of pyritous concretionary distortion, 
but I have seen it in so large a number that I am compelled to regard it as 
a specific character. In some cases the form of every scale of the row is 
observable; it is seen that those immediately back of the head are much 
elongated, and the terminal spine is depressed backward, so that the scale is 
blunt and club-shaped. Possibly this is the result of disease, but if so, it 
attacked a majority of individuals. More likely it is a character developed 
by age and only fully shown by those that were quite old at the time of 
their entombment. It is possible also that it is a sexual character; but, by 
whatever cause produced, it is a mark by which, when present, the species 
can be immediately recognized. 
MON XIY——3 
