CRYPTOPLACID^E. 51 



valves pinkish. Length of valves, measured around back of a 

 curled specimen 117, breadth of widest valve 31* mill. 



Japan (no. 61399 U. S. Nat. Mus.) 



Family CRYPTOPLACID^E Dall. 



Elongated or vermiform Chitons, having proportionally small 

 valves; tegmentum of each valve (except the first) divided into two 

 latero-pleural areas and a dorsal area. Insertion and sutural plates 

 strongly drawn forward, sharp, smooth, the anterior valve with 3-5 

 slits, the other valves with one slit on each side or none. Posterior 

 valve having the mucro far posterior, insertion plate continuous 

 behind, not sinused nor slit there. Girdle very thick and wide, 

 spiculose, generally with small sutural tufts and four around the 

 head valve. Gills occupying the posterior third of the parapodial 

 grooves. 



This family is evidently a comparatively modern branch from the 

 Acanthochitoid stock, differing in the degeneration of the valves in 

 size, consequent upon the adoption of a life in burrows and holes. 

 The number of slits is greatly reduced; and the insertion-plate of 

 the tail-valve has no sinus or upward wave behind. The short gill- 

 row is an inheritance from the Acanthochitidce, which in turn 

 inherited this feature from the low Ischnoid or high Lepidopleuroid 

 stock from which they sprung ; short, posterior gills being char- 

 acteristic of the lowest Chiton stocks, as well as of the Aplacophora. 



The zoological rank of the Cryptoplacidce has been ably discussed 

 by Haddon (Challenger Polyplacophora p. 46, 47), who concludes 

 that "the genus Cryptoplax is a highly specialized branch of a low 

 group of Chitons." To this it should be added that the specializa- 

 tion has been in the direction of degeneration ; the gills are shorter 

 than in the parent stock Acanthochitidce ; the foot and valves are 

 notably reduced in size and functional capacity, and the nervous 

 system shows unmistakeable traces of reversion. 



Two genera, not very diverse in characters, are distinguishable : 



CRYPTOPLAX Blainv., in which the body is vermiform, the 

 anterior valve having 3 slits, the others none ; valves disjointed or 

 merely touching. 



CHONEPLAX Cpr., more like an ordinary Chiton, but much 

 elongated, the valves all strongly overlapping or imbricating. 



