6 ACANTHOCHITID^. 



Family ACANTHOCHITID^ Pilsbry. 



Chitons in which the valves are more or less immersed in the 

 smooth or hairy (never scaly) girdle ; the tegmentum or outer layer 

 therefore being much smaller than the articulamentum or inner layer, 

 and having the exposed surface (when present) divided into dorsal (or 

 jugal) and latero-pleural areas, the latter formed by the union of the 

 lateral areas with the sides of the central areas. Insertion teeth 

 sharp, nearly smooth. Body not vermiform. Posterior valve either 

 slit similarly to the head-valve or having a posterior median sinus ; 

 the mucro submedian. 



Besides the positive and negative characters given above, the species 

 of this family generally have 5 slits in the head-valve, and median 

 or short gills. 



It is difficult to quote synonymy for the family name, as genera 

 grouped here by me are scattered throughout the two grand divisions 

 of the Carpenterian arrangement, being included in the Ischnoidea, 

 Aeanthoidea, Mopaloidea and Cryptoidea of his classification. 



The descent of this family from the primitive Ischnochitonidce can 

 safely be affirmed, although no existing genus affords a clue to the 

 exact branch of that family which gave rise to this peculiar series- 

 The general prevalence of a short gill-row, the simplicity of the 

 insertion-plates and teeth, and the low development of sense-organs 

 in the shell, all indicate an ancestral stock not far removed above 

 the Lepidopleuridce except in the development of slits and teeth. 



The genera of Acanthochitince are closely linked together by inter- 

 mediate forms, although the superficial modification is considerable. 

 The more normal forms (Leptoplax, Spongiochiton) have the tail 

 valve many-slit, like the head valve, and perfectly " regular " in 

 form ; these lead to forms with the posterior teeth uneven and 

 vertical (Loboplax, Notoplax, Katharina), and then to those in which 

 the posterior teeth are obsolete and lost, their place being excavated 

 into a tail-sinus (Acanthochites). So far, the course of development 

 has been parallel to that followed by the Mopaliidce ; but the pro- 

 gressive envelopment of the valves by the girdle, brings another 

 factor into play at this point : viz., the backward growth of the pos- 

 terior covered margins of the valves. This tendency is very clearly 

 seen in the more covered species of Acanthochites, etc., but it becomes 

 much more pronounced in such forms as Cryptoconchus and Amicula; 

 and in Cryptochiton the development of posterior lobes, as well as the 

 burying of the valves themselves, reaches its culmination. 



