ACM.EA. 23 



somewhat pellucid shell. Texture hard aud brittle. Epidermis 

 exceedingly thin, usually evanescent; translucent, brownish. 

 {Ball) 



Fribiloff Is. to Hakodadi, Japan; Aleutian Is.; southeast to 

 Ghirikoff Island. 



A. sybaritica Dall, Amer. Journ. Couch, vi, p. 257, t. 17, f. 34, 

 1871 ; Proc. U. S. Nat. Mus. i, p. 341. 



A beautiful species. The largest specimens attain one inch in 

 length, but those before me measure scarcely over 10 mill. It is 

 always much depressed. Inhabits rather deep water. 



A. PERAMABiLis Dall. PI. 33, figs. 80, 81, 82. 



Shell thin, delicate, ovate ; externally of a uniform dark-rose-color, 

 with a few scattered irregular blotches of light or dark-brown, 

 nucleus pale. Within polished, bluish-white, with a chestnut-brown 

 spectrum with sharply defined edges, outside of which for a short dis- 

 tance the white is unsullied, but further toward the margin in adult 

 specimens, radiating brown blotches may be observed forming a more 

 or less interrupted band around the shell, which is wanting in the 

 young. The margin is of the same deep rose as the exterior. Shell 

 moderately elevated, with the apex well marked, sub-acute and situa- 

 ted in the central third. Nucleus smooth, pale, sharply decurved 

 with a chink beneath it, in front. Sculj^ture of fine, sharp, elevated 

 threads which extend from the vertex to the margin without bifur- 

 cation. These are crossed by very fine sharp lines of growth slightly 

 elevated. 



Length 103 in. lat. 0-8, in. alt. 0-33 in. Posterior slope slightly 

 arched. {Dall.) 



Shumagin group of islands ; A laska Territory, on rocks near low 

 water mark. 



This lovely species has no relations with A. sybaritica Dall 

 and rosacea Cpr., except those of color. The two latter are much 

 smaller and the rose color is much lighter and differently disposed. 

 Its nearest allies are some varieties of A. patina, in none of which 

 have I observed any approach to the color of this species, and 

 which have a different nucleus, and the sculpture in slender rounded 

 riblets instead of sharp threads. The shell of patina is also in 

 general much more solid and thick. The animal partakes of the 

 rosy hue of the shell except the margin of the mantle which is 

 furnished with brown dots. It belongs to the subgenus Collisella. 



