ACANTHOCHITES. 9 



p. 28(1829); type Ch. fascicularis Sow. Gen., f. 3. Stectoplax 

 CPR. in BALL, Proc. U. S. Nat. Mus. 1881, pp. 284, 288, 289, 291 

 (1882) ; type S. porrecta CPR. 



Valves partly covered, the anterior lacking radiating ribs; the 

 posterior valve having the insertion-plate with a single slit on each 

 poster o- lateral edge, and a ivide, toothless sinus in the middle behind. 

 Girdle with large, dense tufts of glistening spicules. Type, C. fasci- 

 cularis L. 



In this, the typical section, the tail valve has been further differ- 

 entiated from the primitive type than in the other sections. The 

 hairs of the girdle, and especially the tufts, are more exuberantly 

 developed than in any othe'r group of Chitons. The valves vary 

 greatly in the degree to which they are buried in the girdle. The 

 species are numerous, but owing to the similarity of the sculpture 

 they are very hard to distinguish, even when well described. 



(1) Species of European and African Seas. 



A. FASCICULARIS Linne. PI. 4, figs. 77, 78, 79. 



Shell elongated, moderately convex, more or less distinctly car- 

 inated. Surface dull, varying much in color, "brown, chocolate, 

 orange, yellow, pinkish or red, now and then mottled or streaked 

 with white, pale green or brown." 



Median valves broadly subtriangular (seen detached), the beak 

 slightly projecting, latero-anterior outline of tegrnentum convex. 

 Latero-pleural areas covered with flat or concave granules, which are 

 ovate-oblong or drop-shaped and rather remote. Dorsal areas tri- 

 angular, not abruptly defined at the edges, rather flattened and 

 longitudinally obsoletely striated. Anterior valve granulated, the 

 lower edge of the tegmentum slightly and obsoletely angular. Pos- 

 terior valve with subcentral elevated mucro. 



Interior greenish, often roseate along the cavity. Insertion-plate 

 of posterior valve having between the two slits a small posterior 

 wave or sinus with a slight lobe on each side. 



Girdle moderately broad, more or less closely covered with short 

 spicules which are usually tawny or grayish ; and a thick tuft of 

 greenish or whitish bristles at each suture, four such tufts around the 

 head valve. Periphery of girdle fringed with spines longer than 

 those covering the rest of the girdle, but shorter than those of the 

 tufts. 



