PATELLID®. 335 
HELCIONIScuS, Dall. Shell solid, heavy, moderately elevated, 
with a subcentral inconspicuous apex. Branchial cordon inter- 
rupted in front, ending abruptly on each side, at the anterior 
ends of the adductor; sides of foot and mantle-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 front; teeth with the inner two series parallel, 
third series with a larger denticulate cusp, posterior, three uncini 
on each side the radula. P. pellucida, Linn. (Ixxxiv, 51). 
ANCISTROMESUS, Dall. Shell white, apex subcentral, obsoletely 
radiately ribbed. Animal blackish, with complete branchial 
cordon, the lamelle 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, Mexicanus, 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, ete. 
METOPTOMA, Phillips. (Tryblidium, Lindstrim, 1880.) Shell 
like Helcion, with a broad but slightly or obsoletely marked 
scar below the apex, truncate or somewhat insinuated on the 
margin. Paleozoic; 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 insufliciency of their distinctive characters ; 
like him I have placed them in the synonymy—but give their 
descriptions, 
SCUTELLASTRA, H. and A. Adams. Shell coarsely ribbed, the 
ribs causing long projections of the margin. P. longicosta, Lam. 
(Ixxxiii, 26, 27). 
CYMBULA, H. and A. Adams. Shell oblong, laterally com- 
pressed, with radiating striz and recurved apex. P. compressa, 
Linn. (1xxxiii, 28). 
OLANA, H. and A. Adams. Shell contracted in front; apex 
obtuse. P. cochlear, Born. (1xxxiii, 29). 
Leperopsis, Whitfield, 1882. 
Type.—L. Levettit, 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 (?)-m-‘front; consisting ‘of ‘anirregular*harrow band which 
