402 



POLYPLACOPHORA 





diverge from the neighbourhood of the median heak or umbo. 



The space enclosed between these ribs is known as the median 



area or jugum, the other two 

 spaces as the lateral areas or 

 ■pleura. The ribs terminate with 

 the edge of the tegmentum, and 

 are not found on the articula- 

 mentum. In certain genera these 

 areas are either non-existent, or 

 are not distinctly marked. The 

 sculpture of tlie lateral areas (which 

 is, as a rule, much stronger than 

 that of the median area) will 

 generally be found to resemble 

 that of the anterior valve, which 

 has no proper median area. In 

 the posterior valve the median 



Fig. 263. -First, fourtli, and eighth area is very small, while the sculp- 

 valves of a Chiton, showing Li, ture of the rest of the valve cor- 



laminae of insertion ; n, n, notches ; i j. i 



5.^, 5.Z, suturai laminae, x 2. rcsponcls to that ol the lateral 



areas generally (see Fig. 261). 



The articulamentum of the intermediate valves is divided into 

 two equal parts in the middle of the anterior edge, opposite to 

 the beak, by a sinus. Each of the portions thus formed is again 

 divided by a notch or suture into two unequal parts, the anterior 

 of which is known as the sutural lamina, and is more or less 

 concealed by the valve in front of it, while the lateral part, or 

 lamina of insertion, is entirely concealed by the girdle. The 

 articulamenta of the anterior and posterior valves are either 

 simple or pierced by a series of notches (Fig. 263). 



The girdle of the Chitonidae varies considerably in character. 

 Sometimes its upper surface is simply corneous or cartilaginoid, 

 with no other sculpture than fine striae, at others it is densely 

 beset with spines or bristles, or tufted at intervals with bunches 

 of deciduous hairs ; again it is marbled like shagreen or mossy 

 down, or covered with serpent-like scales. The width of the 

 girdle varies greatly, being sometimes very narrow, sometimes 

 entirely covering all the valves {Cryptochiton). As a rule, its 

 outer edge is continuous, but in Schizochiton it is sharply notched 

 over the anus. 



