EEPOKT ON THE POLYPLACOPHORA. 43 



are characterised by the last four valves being considerably isolated, the distance between 

 them varying with the contraction or extension of that region of the body. On this sub- 

 ject M. de Rochebrune says : " La position isolee des valees a ete tout aussi mal interpre'te'e ; 

 douze especes de Chitonelles et soixante et un individus de ces especes sont sous nos 

 yeux, toutes a. l'exception des Cryptoplax fasciatus, Quoy et Gaim., et oculatus, Quoy 

 et Gairn., out les valves inibriquees, et meme ces deux especes n'ont de reellernent isole'es 

 que les trois valves posterieures." 1 



The valves of Cryptoplax consist of a small tegmentum resting like a cap upon the 

 well-developed articulamentum. The articularnentum may perhaps be best described, 

 for all the valves except the first, as resembling a heart-shaped or sagittate scoop, the emar- 

 gination being in front. The posterior border is always entire and often greatly thickened, 

 and it may be squared, rounded, or pointed ; the bowl of the scoop may be comparatively 

 flat, or the two sides may meet at a sharp angle. The anterior wings constitute the laminae 

 of insertion of the valve, and are unslit at the sides or behind in the terminal valve. 



The anterior valve is oval in shape, but more or less squared posteriorly ; the articula- 

 mentum is also scoop-shaped, and its posterior border may be enormously thickened ; the 

 anterior projection forms the lamina of insertion of this valve, which here alone is slit, there 

 being three slits, forming two central broad teeth. The tegmentum may leave a larger or 

 smaller amount of the articulamentum exposed on a dorsal view, and on the under surface 

 it is seen to wrap round the hinder edge of the articulamentum, as is usually the case in 

 other Chitons. The upper surface of the valve is usually much corroded, and its sculpture 

 generally appears to consist of irregular granules and to differ slightly from that of the 

 other valves. 



The tegmentum of the remaining valves has a more or less pyriform or oval contour ; 

 it always leaves the lamina? of insertion largely exposed ; posteriorly it usually forms a 

 sort of cap or clothing to the apex or rounded end of the articulamentum, and thus is 

 generally apparent on an under view of the valve, where it has some resemblance to the 

 calyx of a flower. In all the valves the jugum is distinguished as a more or less smooth 

 elongated triangle, the pleura being granular and irregularly grooved, the grooves radiating 

 from the posterior umbo. There are no lateral areas. 



The above general account will serve to explain the drawings on Plate III., and on refe- 

 rence to these the differences between the valves of the one species or between corres- 

 ponding valves of the four species will be readily apparent, so that there is no need to 

 enter into minute description or comparison of the several valves. 



The first three and the last valves have alone been figured, as differences between the 

 remainder and the third are of no great importance or interest ; each of these valves has 

 been drawn as seen from above, from below, and from the side. The first three species 

 are drawn from specimens collected by the Challenger Expedition. Mr Smith kindly 



1 Ann. des Set. geol., vol. xiv. p. 11. 



