ACANTHOPLEURA. 219 



Girdle broad, fleshy, beset with strong spike-like spines. The 

 gills extend the whole length of the foot. 



Length 100, breadth 55 mill. ; divergence 115-120. 



Length 63, breadth 40 mill. 



Payta, Peru (Dr. W. H. Jones) ; Galapagos (P. Ac. Coll.) ; Val- 

 paraiso, Chili (Frembly.) 



Chiton echinatus BARNES, Amer. Journ. Science and Arts, (Silli- 

 man's) vii, p. 71, t. 3, f. 4a, 46. (1823). SOWERBY, Conch. Illustr., 

 f. 47 (young). Chiton tuberculiferus SOWERBY, Cat. Tankerv., p. 

 29, no. 688 (1825). Chiton spiniferus FREMBLY, Zool. Journ. iii, 

 p. 196, Suppl., pi. 16, f. 1 (1827). Chiton aculeate REEVE, Conch. 

 Icon., t. 9, f. 49. Not C. aculeatus of DESK, et al, nor of Linn. 

 Chiton ( Corephiwn) spiniferus Frembl., STEARNS, Proc. U. S. Nat. 

 Mus. xiv, 1891, p. 334. 



This large, spiney species is not likely to be confused with any 

 other, being very distinct in external features as well as in the 

 peculiar insertion-plate of the tail-valve, the engraving of the inte- 

 rior (reminding one of Enoplochiton niger and Chiton granosus), and 

 the denticulate sinus. The girdle spines are frequently encrusted 

 with a white calcareous deposit, converting them into globular 

 pellets. The young shell also lacks slits in the posterior insertion- 

 plate (figs. 15-17, x 2) ; and the adults sometimes show an addi- 

 tional shallow notch or two near the median caudal slit. 



Subgenus ACANTHOPLEURA, s. sir. 



Aeantlwpleura GUILDING, I. c. (part), and PILSBRY, The Nauti- 

 lus vi, p. 105, Jan. 2, 1893. Francisia CARPENTER in DALL, 

 Proc. U. S. Nat. Mus. 1881, p. 284, 289, 290 (1882.) 



Acanthopleura : Tail valve having a long, many-slit insertion- 

 plate, crenulated outside ; median valves much wider across the 

 sutural-plates (or articulamentum) than across the exposed portion 

 (tegmentum.) 



The facts which cause me to consider this the typical group of 

 Acanthopleura have been discussed above. The affinities of the sub- 

 genus are with Maugeria and with Mesotomura. With the latter it 

 agrees in the coarsely spiney girdle, interior coloring, denticulate 

 sinus and long insertion-plates; but it differs from that group in 

 having a many-slit tail-plate, in the frequent (or usual) plurality of 

 side-slits, the nearly smooth exterior, etc. In the form and den tic- 



