76 PATELLID^E. 



more or less warped, owing to its habitat ; edge thin and simple. 

 Sculpture none, lines of growth slight, outer surface dull white; 

 inner surface smooth, with the pallial markings faint. 



Length 3'75, breadth 3, alt. 2 mill. (F.) 



Off Martha's Vineyard ; Gulf of Mexico between the delta of the 

 Mississippi and Cedar Keys, Fla., 130-388 fms. 



L. tubieola V. & S. Amer. Jn. Sci. 1880, p. 396 ; Proc. U. S. Nat. 

 Mus. 1881, p. 375 ; Trans. Conn. Acad. v, p. 534, t. 58, f. 29, 29a 

 BALL, Proc. U. S. Nat. Mus. iv, p. 408 ; Blake Gastrop., p. 413, t. 

 25, f. 6 (Dentition.) 



Young specimens show that the nucleus is subspiral, as in other 

 Lepetidce. 



Family PATELLIDJE. 



Docoglossate gasterpods having a simply conical shell, non- 

 spiral even in the embryo. Breathing by a cordon of branchial leaf- 

 lets attached to the mantle between its thickened edge and the sides 

 of the foot; having no cervical gill-plume. Radula having three 

 uncini and three laterals on each side, the rhachidian tooth being 

 either present, rudimentary or wanting; jaw developed. 



The Patellidce differ markedly from Acmceidce and Lepetidce in 

 the gills, which form a complete or interrupted cordon, not accom- 

 panied by a cervical branchial plume, and not homologous with the 

 gills developed in other Prosobranchiata. 



The shells may generally be distinguished from those of the 

 Acmceidce and Lepetidce by their texture and the lack of a defined 

 internal border; but the distinction is difficult or impossible to ex- 

 press in words, and must be learned by actual familiarity with 

 the objects themselves. 



In the arrangement of the species and groups I have made use of 

 the character of the texture of the interior, heretofore neglected by 

 systematists, but undoubtedly of equal importance in many cases for 

 the discrimination of groups with the character of the gill-cordon 

 and the presence or absence of a rhachidian tooth. 



The rhachidian tooth is now proven to be decidedly variable in 

 closely allied species. It is well-developed and bears a cusp in 

 Ancistromesus and many species of Scutellastra ; is represented by a 

 linear rudiment in Patina, Nacella, Patinella and Helcioniscus. It 

 is apparently wanting in 'Patella s. str. 



