PATELLID^. 335 



HELCiONiscus, Dall. Shell solid, heavy, moderately elevated, 

 with a subcentral inconspicuous apex. Branchial cordon inter- 

 rupted in front, ending abruptl}^ on each side, at the anterior 

 ends of the adductor ; sides of foot and niantle-edge smooth ; 

 inner uncinus hardly raised above the level of the lingual ribbon, 

 second lateral tooth largest. P. exaratus, Nutt. (Ixxxiii, 25\ 



PATINA, Leach. (Nacella H. and A. Adams, in part. Ansates, 

 Sowb.) Shell with the apex subterminal, anterior; smooth, 

 thin, semipellucid or horny. Animal with the branchial cordon 

 interrupted in fi'ont ; teeth with the inner two series parallel, 

 third series with a larger denticulate cusp, posterior, three uncini 

 on each side the radula. F. pellucida, Linn. (Ixxxiv, 51). 



ANCISTROMESUS, Dall. Shell white, apex subcentral, obsoletel}'^ 

 radiatel}^ ribbed. Animal blackish, with complete branchial 

 cordon, the lamellee being long and slender, subequal ; sides of 

 foot smooth ; radula with a simple rachidian tooth, the two 

 inner laterals on each side anterior to the third pair, which are 

 large and quadridentate, uncini simple. 



A. 3Iexicanus, Brod. is the largest living limpet, the shell 

 attaining a length of from 8 to 14 inches. It is frequently used 

 as a wash-basin in Central America, and inhabits the West Coast, 

 extending northwards to Acapulco, etc. 



METOPTOMA, Phillips. (Tryblidium, Lindstrlim, 1880.) Shell 

 like Helcion, with a broad but slightly or obsoletely marked 

 scar below the apex, truncate or somewhat insinuated on the 

 margin. Palaeozoic; United States, Europe. P. Solaris, Kon. 

 (Ixxxiv, 52). 



The following subgenera of Patella occur in the work of 

 Messrs. H. and A. Adams. They are not recognized by Mr. 

 Dall on account of the insufficiency of their distinctive characters ; 

 like him I have placed them in the synonymy — but give their 

 descriptions. 



SGUTELLASTRA, H. and A. Adams. Shell coarsely ribbed, the 

 ribs causing long projections of the margin. P. longicosta^Jjixm.. 

 (Ixxxiii, 26, 27). 



CYMBULA, H. and A. Adams. Shell oblong, laterally com- 

 pressed, with radiating stria and recurved apex. P. compressa, 

 Linn. (Ixxxiii, 28). 



OLANA, H. and A. Adams. Shell contracted in front ; apex 

 obtuse. P. cochlear. Born. (Ixxxiii, 29). 



Lepetopsis, Whitfield, 1882. 



Type. — L. Levettii, White. Carboniferous ; Indiana. 



Shell patelliform, more or less regularly round or oval, apex 

 subcentral, posterior to the middle and directed backward, the 

 nucleus dextrally coiled; muscular imprint horseshoe-shaped, 

 open (?) in front, consisting of an irregular narrow band which 



