182 COMPARATIVE ZOOLOGY. 
side of a shell mark the successive periods of growth, and, 
therefore, correspond with the age of the animal. The 
following figures show the principal parts of the ordinary 
bivalves and univalves. The valves of a bivalve are gen- 
erally equal, and the umbones, or beaks, a little in front 
of the centre. The valves are bound together by a liga- 
ment near the umbones, and often, 
also, by means of a “hinge” form- 
ed by the “teeth” of one valve in- 
terlocking into cavities in the oth- 
er. The aperture of a univalve is 
Fig. 99.—Left Valve of a Bivalve Mollusk (Cytherea Fria. 100.—Section of a Spiral 
chione): h, hinge ligament; uv, umbo; J, lunule; Univalve (Triton corrugatus) : 
c, cardinal, and ¢, t’, lateral, teeth; a, a’, impres- a, apex; b, spire; c, suture; 
sions of the anterior and posterior adductor mus- d, posterior canal; e, outer 
cles; p, pallial impression; s, sinus occupied by lip of the aperture; J, anteri- 
the retractor of the siphons. or canal. 
frequently closed by a horny or calcareous plate, called 
“operculum,” which the animal carries on its back, and 
which is a part of the exoskeleton. 
Imbedded in the back of the Cuttle-fish is a very light 
spongy “bone,” which, as already observed, is a secretion 
from the skin, and, therefore, belongs to the exoskeleton. 
It has no resemblance to true bone, but is formed, like 
shells, of a number of calcareous plates. Nevertheless, 
the Cuttle-fish does exhibit the first traces of an endo- 
