r> ACANTIK TIN T I DJE. 



Family ACANTHOCHITID^ Pilsbry. 



Chitons in which the va more or less immersed in the 



smooth or hairy (uever scaly) girdle ; the tegmentum or outer layer 



t'.iv being moob smaller than the articuhmicntuin or inner layer, 



and havini: 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 



:>, nearly smooth. Body not vermiform. Posterior valve either 



.-lit similarly to the head-valve or having a posterior median sinus : 



the mnero sulmiedian. 



Besides the positive and negative characters given above, the species 

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

 <>rt 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, 

 Acantlto ,doidea 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 / (u-idcR 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, iSpongiochiton) 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 (Ijoboplax, Notophu, Kath<irina\ and then to those in which 

 tin- jx.sterior teeth are obsolete and lost, their place being excavated 

 int.. a tail-sinus (Acanthochitet). So far, the course of development 

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

 gressive envelopment of the valves by the girdle, brings another 

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

 terior covered margins of tin- valve-. Thi< tendency is very clearly 

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

 much more pronounced in surh forms as Cryptoconch"* and Amirnhi ; 

 and in l'r<ij.trl,itim the development of posterior lnl,-. as well as the 

 huryiiii: of the valves themselves, readies its culmination. 



