DAVIS: NOTE ON CHLAMYDOSELACllUS. 
103 
head spi eads easily till its width equals its length, and the outline 
from above resembles an equilateral triangle, or. better, an arrowhead 
with barbs. The gape is wide. Both mouth and throat are lined 
with shagreen. On the inner edges of the gill arches the scales are 
larger. At the angle of the jaws there are neither labial folds nor 
labial cartilages. The eye is moderately large ; it is on the side of 
the head, over the middle of the length of the jaws, and from the 
sharp, rather })romineut brow, has a savage look There is no 
nictitating membrane, but around the pupil the skin covering the 
eyeball is rough with small scales. The snout extends but little in 
advance of the mouth. The nostrils are lateral : placed about half- 
way between the eyes and the snout similar in structure to Notidanidee. 
The gill-openings are large, oblique ; arches slender. The opercular 
flap or first gill cover is broad and free around the neck except for a 
short space behind the occiput. 
An open canal, the lateral line, extends on each side from the 
back of the skull to the end of the tail ; other open canals, branches 
of the same system, ramify in several directions over the head. The 
pectoral are separated from the ventral fins by a distance of twenty- 
two inches : at this part of the body the height is four inches, the 
width three inches. A i)rominent double or grooved keel along the 
median abdominal line adds considerably to the de[)th, in the centre 
of the body it projects three-quarters of an inch. From their position, 
shape, and extent, it is considered that the folds Avill furnish support 
to one of the theories of the origin of paired fins. The muscle of 
the inside of the keel corresponds to the rectus abdominis of other 
vertebrates. 
Mone of the tins are rigid but on the contrary very soft, and 
like the body itself extremely flexible. They are covered with 
shagreen except near the outer edges, which are thin and membranous. 
The dorsal is single, comparatively small, five-and-a-half inches in 
length, opposite to the anal. Its anterior origin is indicated by a 
peculiar armature. The pectoral, ventral, anal, and candal fins are 
large, The pectorals are moderately long, 5-75 inches, both margins 
are curved meeting in a blunt angle at the end of the fin. The 
ventrals are placed some distance behind the middle of the total 
