290 On a Natural Arran<jeinent of the Order Docoglossa. 



Type Ilelcion ]:)ectinatus (as Patella pectinata)^ Linn.^ 

 Gmel. Syst. Nat. 1792, p. 3710. no. 93. 



2. Helciomscus, Dall, n. subg. prov. Shell depressed, solid, 

 with a subcentral apex ; teeth arranged as in Pattnella; 

 sides of foot smooth. 



Formula, 3(2-1° 1-2) a ' 



Type Helciomscus rota (Chemn.), Rve. (as Patella) 

 Conch. Icon. pi. 17. fig. 39, a, &, c. 



^.Patina (Leach), Gray, 1840. Shell very thin, pellucid; 

 sides of foot smooth ; third pair of laterals posterior, 

 largest, denticulated. 



Formula, ^^,_^\_,)^ ' 



Type Patina jyellucida, Linn. Syst. Nat. cd. 12, 1767, 

 p. 1260. no. 770 {sls Patella) . 



Soft parts? 



1. Metoptomaj Phillips, 1836. Shell ovate, triangular, apex 

 subcentral, posterior end truncated, or deeply, broadly 

 emarginated. 

 Type MetojHoma pileus^ Phil. Geol. Yorkshire, vol. ii, 

 p. 223 (1836). Fossil in the Carboniferous formation 

 of Great Britain. Many of the species referred to this 

 genus by Billings and other paleontologists clearly do 

 not belong to it. 



The above sections, with the exception of Helcion^ are well- 

 defined, and will probably include the greater portion of the 

 known species, though some may prove distinct from any 

 yet examined. Extensive study of the soft parts has shown, 

 beyond dispute, that generic distinctions founded on the shells 

 alone are wholly valueless, as the latter cannot be depended 

 upon for diagnostic characters ; and many so-called genera and 

 subgenera founded upon the shells will fall as synonyms, or 

 retain their places solely as the result of accident. Scutellina^ 

 as far as known, is equivalent to Aoncea. Olana., Scutellastra^ 

 Cellana, &c. are founded upon characters of hardly specific 

 value. The results of extended researches on this order are 

 now in the press, which will include a thorough revision of 

 the synonymy in full, with a definite reference of many spe- 

 cies to their proper position, as determined by the sum of all 

 their characters. 



