PATELLIDvE. 79 



Synopsis of Groups of Patellidce. 



A. Two inner lateral teeth on each side anterior. 



Subgenus PATELLA Linne, 1758 (restricted.) 



Branchial cordon complete; sides of foot having no epipodial pro- 

 jections. Two inner lateral teeth on each side anterior, the rhachidian 

 tooth present or absent. Apex of the shell near the center. Type 

 P. 'vulgata. 



Section PATELLA s. sir. Inner layer of the shell subtranslu- 

 cent, exhibiting when closely examined a concentrically 

 fibrous texture ; more or less iridescent. Radula without 

 a rhachidian tooth ; formula 3 (i 202 i) 3 . Type P. vulgata. 

 Cymbula Ads. and Patellastra Monts. are synonyms. 

 Section SCUTELLASTRA Ads., 1858. Inner layer of the shell 

 opaque, porcellanous, not iridescent. Radula either with 

 or without a rhachidian tooth ; formula 3 (i 202 i) 3 or 3 (i 2I2 i) 3 . 

 Olana Ads. is a synonym. 



Section ANCISTROMESUS Dall, 1871. Inner layer of the large, 

 massive shell porcellanous, opaque. Rhachidian tooth of 

 the radula developed, bearing a cusp ; formula 3 (i 212 i) 3 . 

 Type P. mexicana. 



Subgenus HELCION Montfort, 1810. 



Branchial cordon interrupted in front ; side of foot smooth, with- 

 out epipodial processes. Inner two lateral teeth on each side ante- 

 rior, no rhachidian tooth. Shell oval, apex anterior. Formula of 

 teeth 3 ( 1 * 1 )s. 



Section HELCION s. str. Shell oval, with anterior apex ; sur- 

 face sculptured with scaly radiating ribs. Type H. pect- 

 inatus. 



Section PATINA (Leach) Gray, 1840. Shell with anterior or 

 subcentral apex ; radiately striated, polished. 



B. One inner lateral tooth on each side anterior. 



Subgenus NACELLA Schumacher, 1817. 



Branchial cordon complete; sides of foot bearing a scalloped 

 epipodial ridge. One inner lateral tooth on each side anterior; 

 rhachidian tooth none or rudimentary. Interior of shell having a 

 satin-like or metallic luster. 



