epidermis none ; sculpture externally of fine, uniform, rounded, 

 closely set threads, radiating from near the apex to the margin and 

 reticulated by the fine, rather prt)minent, regular, concentric ridges 

 of growth, both ridges and threads averaging near the margin about 

 three and a half to the millimeter. Length., from end to end, 14-5 

 mm. ; from apex to anterior end 5*5 mm. ; lat. lO'O mm. ; alt. 5*5 mm. 

 {Ball.) 



Off St. Lucia, 226 fms. ; off Dominica, 333 fnis. ; off Gnadelupe, 

 583 fms.; and off St. Thomas. 



P. arcuata Ball, Proc. U. S. Nat. Mus. 1881, p. 409, 1882 ; Blake 

 Rep. p. 411, t. 25, f 3,3a, 3b. 



Subfamily AcM^iNiE. 

 Genus ACM^A Eschscholtz, 1830. 



Acmcea EscH., in append. Kotzebue's Neue Reise, ii, p. 24, 1830, 

 type A. mitra Esch. — Forbes & Hanley, Brit. Moll., ii, p. 433. — 

 Cpr., Mazat. Catal. p. 202. — Dall, Amer. Journ. Conch, vi, p. 

 237. — Watson, Challenger Gastr. p. 28. — Fischer, Manuel, p. 

 865. — Tecture Aud. & Milne-Edw., in Cuvier's Rapport sur trois 

 Memoires, etc., Annales des Sci. Nat. xxi, 1830, p. 326, published 

 not before 1831, type P. virginea. — Tectura of Gray, H. & A. 

 Adams, Jeffreys, ef al. — Patelloidea Quoy & Gaimard, Voy. 

 Astrol. iii, p. 349. Type P. fragilis (Chemn.) Q. & G., 1834.— 

 Lottia Gray, in pare, Philos. Trans. 1833, p. 800. — Lottia of Gould, 

 et al. — Erginus Jeffreys Ann. Mag. N. H. 4th ser., xix, p. 231, 

 March, 1877. Type Tectura rubella Fabr. — Collisella Dall, Amer. 

 Journ. Conch, vi, p. 245, 1871. Type .4. pelta Esch. — Collisellina 

 Dall, I. c, p. 154, type A. saccharina. L. 



Shell conical, patelliforra, apex more or less anterior. Animal 

 with a branchial plume at the left side of the neck above ; no 

 branchial cordon. Dentition, see below. 



The shells may generally be distinguished from Patella by the 

 different texture and marginal border of the inside. 



The thorough discussion of the generic name of this group con- 

 tained in the various papers of Dr. Dall and others, renders any 

 justification of the view of its nomenclature here taken, unnecessary. 



Tectura and Erginus must be regarded as absolute synonyms of 

 Acmcea s. str. ; Patelloidea Q. & G. will probably be found to diflfer 

 somewhat anatomically, and may then be utilized for a subgeneric 

 group. The subgenera Collisella and Collisellina are defined below. 



