294 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



DOCOGLOSSA. 



ACM^EIDiE. 



Genus ACMJEA, Esch. 



Synon. — Patella, Beleion, Scurria, Pileopsia, &c. (sp.), of authors; but uot as properly understood. 



Acmcea, Eschscboltz(1828),App. to Kotzebue's New Voy. around tbe World; and (1830) in Cobum 



and Bentley* Loud, transl. same, II, 350 ; also (1833) Zotil. Alias (ed. Katbke), 16 — 



Fcrbes aud Hanley (1853), Brit, Moll., II, 433.— Dall (1870), Am. Jour. Concb., VI, 237. 

 Teoture, Audouin and Edwards (1830), Ann. Sci. Nat., XXI, 326; and (1832) Rdeh. Hist. Nat. 



Littor. Fr., I, 144. 

 Patelloidea (Quoy and Gaimard, MS.), Deshayes (1832), Enoyo. Melh., Ill, 704.— Quoy and Gaim. 



( 1833), Voy. Astrolabe, III, 349 ; and.of others. 

 Lottia, Gray (1833), Phil. Trans., 123 aud 800 ; also (1840) Synon. Moll. Brit. Mus., * * .—Forbes 



(1838), Malac. Mom, 34.— Reeve (1841), Proceed. Zool. Soc., 75 ; and (1842) Concb. 



Syst., II, 17.— Miiller (1842), Index Moll. Grcenl., l(i; and of many others. 

 Tectum, Gray (1847), Proceed. Zool. Soc., 158.— H. and A. Adams (1854), Genera Recent Moll., I 



458.— Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 17.— Dall (1870) 



Proceed. Bost. Soc. N. Hist., 245 : and of others. 



ICtym. — d/c/ta'oc, blooming. 

 Type. — Acmcea mitra, Esch. 



Shell patelliform, generally depressed, regular, thin, smooth or radiately 

 striated; base subcircular or oval; apex antero-ceetral, or more or less in 

 advance of the middle, obtuse, with usually a slight forward obliquity ; aper- 

 ture very large; muscular impression horse-shoe-shaped, interrupted ante- 

 riorly. 



My friend Mr. Dall, to whose excellent paper on (his and allied groups 

 (published in the Journal of Conchology) I am indebted for the foregoing 

 synonymy, divides this genus into two subgenera; that is, into Acmcea 

 proper, and the subgenus Collisella; but these distinctions, being based on 

 peculiarities of the soft parts and the dentition, of course cannot be made 

 available in palaeontology. 



Indeed, among fossil species, it is very difficult, and perhaps often impos- 

 sible, to distinguish this genus from Patella. Most generally the shell in 

 Acmcea is more depressed and thinner than in Patella; and, as noticed by 

 Forbes and Hanley, the recent shells of the former are, to some extent, dis- 

 tinguished by "an absence of a nacreous gloss on the upper surface." As 

 conchologists mainly rely on differences in the animals of these two genera, 

 however, it may readily be understood that the task of the palaeontologist, in 

 attempting to separate them among fossil species, is very difficult. Con- 

 sequently, the geological range of Acmcea is far from being well deter- 

 mined. Species undistinguishable, however, by any known characters, occur 



