336 CHITONID^. 



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 POLYPLACOPHORA. 



Animal symmetrical, with a broad foot ; no eyes or tentacles ; 

 head extensible into a proboscis ; mouth furnished with jaws and 

 lingual ribbon ; branchiae posterior, marginal, between the 

 mantle and foot ; heart median ; intestine straight, anus pos- 

 terior ; sexes united in the same individual. 



Shell when present multi valve, consisting of eight separate 

 pieces inserted upon the back of the animal and surrounded by 

 a mantle-border. 



Family 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 (arese 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 (arese centrales) are sculptured alike, but generally in 

 a different pattern from the sides. The arese 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-laminse of insertion, which project into the 

 zone or girdle, and are free from the peculiar porous superficial 

 laj^er characteristic of the exposed test in the whole group of 

 Chitons. This superficial layer usually projects over the anterior 

 and posterior laminae of insertion or teeth (dentes) in the first 

 and last valves, forming what Dr. Carpenter terms the " eaves " 

 (subgrundse). These may exhibit the spongy character of the 

 layer of which they are formed, or ma}^ be varnished over at 

 their edges with a thin la3^er of true shell}' 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 a small 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 of the two anterior sutural laminae which 



