MUSEUM OF COMPARATIVE ZOOLOGY. 7 
broader bases. These resemble the teeth of certain fossil species which 
have single cusps. They are hardly one fourth as large as the teeth 
immediately in front of them. Where they have been worn, on the top 
of the head or on the belly, the scales are not so harsh to the touch. 
From each side of the lateral line elongate scales with chisel-shaped or 
truncate ends reach out to meet similar ones from the other side, thus 
forming a cover or protection for the canal (fig. 10). The upper edge 
of the tail and its posterior border, to the lateral line, are armed by a 
sharp edge of scales. The edge is formed of two rows — one from each 
side — of broad, thin, subquadrangular scales, which have met on the 
median line and become so closely applied as to appear a single ridge. 
Each scale entering into the construction of the edging is opposed to 
two others, in this manner imbricating or breaking joints. Near their 
bases these plates are striated ; their distal halves are smooth. Similar 
scales guard the front or upper edge of the dorsal. 
Comparing the scales with those of a very young Heptabranchias pec- 
_torosus, we find that in the latter the shapes and sizes are much more 
regular, that all are three-cusped, and that on the upper edges of tail 
and dorsal there are placed side by side three series of enlarged and de- 
pressed scales. On a specimen of Heptabranchias cinereus, thirty-seven 
inches in length, the upper edges of dorsal and tail are covered with 
slightly enlarged scales, which differ little from those of the sides, and 
the scales on the lateral line — which has a dermal cover —are not en- 
larged or different in any way from those of other parts of the body. 
The line itself ends, m that species, before reaching the notch in the 
caudal; it is only to be traced by its pores. On a large H. maculatus 
the lateral line of the scapular region is alternately open or closed for 
irregular distances. The skin being thin, the canal is shallow or near 
the surface. Along the edges of the open portions the scales differ little 
from the others, 
The Skull. 
Plates VIi. and VIII. 
Between Chlamydoselachus and its nearest allies there are internal 
differences which are quite as numerous and striking as the external. 
The comparative length of the skull, the length of the jaws, and the 
position in which the latter are suspended, again present a remote re- 
semblance to the serpents. From the marked similarity in the brain, 
branchialia, and in other respects, one would not expect great differences 
in the skulls of this genus and the Notidanidae, yet from the skulls 
