336 CHITONIDA. 
expands more or less at the anterior extremities ; surface of the 
shell marked by six very indistinct radiating lines, two anterior, 
two posterior, and two lateral. 
ORDER POLY PLACOPHORA. 
Animal symmetrical, with a broad foot ; no eyes or tentacles ; 
head extensible into a proboscis; mouth furnished with jaws and 
lingual ribbon; branchiz posterior, marginal, between the 
mantle and foot; heart median; intestine straight, anus pos- 
terior ; sexes united in the same individual. 
Shell when present multivalve, consisting of eight separate 
pieces inserted upon the back of the animal and surrounded by 
a mantle-border. 
Famity CHITONID &. 
Shell composed of eight separate transverse imbricating 
plates, lodged in a coriaceous mantle, which forms an expanded 
margin around them. Dentition (xii, 52). 
The following description of the Chitones is illustrated, as to 
the shell, Ixxxiv, 53,55, 56; Ixxxv, 54. 
In all Chitons with exposed valves, the seven posterior valves 
are divided more or less plainly by lines radiating from the apex 
to the opposite anterior edge. The sculpture of the posterior 
triangular areas (are laterales) thus cut off is almost uniformly 
like that of the whole anterior valve and the part behind the 
apex (mucro) of the posterior valve. The central or anterior 
triangles (arez centrales) are sculptured alike, but generally in 
a different pattern from the sides. The arez laterales are usually 
raised a little above the rest. It is very rare that the bounding 
diagonal lines cannot be traced, and they usually correspond to 
the slit in the side-laminz of insertion, which project into the 
zone or girdle, and are free from the peculiar porous superficial 
layer characteristic of the exposed test in the whole group of 
Chitons. This superficial layer usually projects over the anterior 
and posterior laminz of insertion or teeth (dentes) in the first 
and last valves, forming what Dr. Carpenter terms the “ eaves ” 
(subgrunde). These may exhibit the spongy character of the 
layer of which they are formed, or may be varnished over at 
their edges with a thin layer of true shelly matter, as in the- 
Ischnoid group. In the typical Chitons they are short, leaving 
the teeth projecting; in the Mopaloids they are hardly 
developed, and in some groups they quite overshadow the 
teeth. 
In many groups there is asmall portion of peculiar sculpture 
marked off along the ridge of the median line of the back. This 
is the area jugali,and corresponds to the sinus or space between 
the inner terminations ef the two anterior -sutural lamina. which 
