62 GASTROPODA. [Aspidobranchia. 



The shells, which are extremely variable, may be distinguished 

 from Patellidce by their different texture and the more or less 

 distinct internal border of the aperture ; they are never iridescent 

 within. 



Genus 1. ACM^EA, Eschscholtz, 1830. 



Acmcea, Esch., " Appendix to Kotzebue's Neue Reise," vol. ii, p. 24, 

 1830. Type: Acmcea mitra, Esch. Tecture, Aud. & Milne-Ed w., Ann. 

 Sci. Nat., xxi, 1830 (1831), 326. Tectura of Gray, H. and A. Adams, &c. 

 Patelloidea, Q. & G., Voy. Astrol., iii, 349. Lottia (in part), Philos. 

 Trans., 1833, 800. Erginus, Jeffreys, A.M.N.H. (4), xix, 1877, 231. 

 Collisella, Dall, Amer. Journ. Conch., vi, 1871, 245. Collisellina. Ball. 

 I.e., 154. 



Tentacles rather long, cylindrical ; eyes situated on their posterior 

 upper side. Foot semicircular ; ctenidmm large, directed from left 

 to right ; anal orifice on the right side, and near the extremity of the 

 adductor muscle. 



Shell patelliform, usually solid, oval or circular, summit an- 

 teriorly directed and more or less in front of the middle ; interior 

 not iridescent, and generally having an internal marginal border of 

 colour. 



Pilsbry arranges the species into six geograpmcal divisions 

 (1) North Atlantic and Arctic ; (2) western coast of North America : 

 (3) western coast of South America ; (4) West Indies ; (5) Japan ; 

 (6) Lido-Pacific. 



Tertiary to Recent. One species is known from the Cretaceous of 

 America. 



KEY TO SPECIES. 



A. Shell distinctly ribbed, ribs visible to the naked eye. 

 a. Colour white, light brown, seldom greenish. 



b. Ribs 8, shell star-shaped . . . . . . octoradiata. 



bb. Ribs more than eight. 



c. Ribs elevated, roughened. 



d. With 7 primary stouter ribs, margin 



laciniate . . . . . . . . stella. 



dd. With 14-22 subequal ribs, margin den- 

 ticulate . . . . . . . . corticata. 



re. Riblets distinct, but lower, not roughened. 



d. Riblets 10-30, conspicuous, apex at the 

 anterior third, inside usually rayed 

 with pinkish . . . . . . rubiginosa. 



dd. Riblets 30-50, low, apex at the anterior 

 fourth, spatula white, sometimes with 

 a brown spot, inside with a pinkish 

 pinkish border . . . . . . cingulata. 



ddd. Riblets 25-50, sharp, usually no inter- 

 stitial riblets, spatula dark brown . . intermedia. 

 <KI* Colour pink, numerous close riblets . . . . Campbelli. 



aaa. Colour brown, usually tessellated with white or green, 



riblets delicate or distinct and granulose . . . . septiformis. 



