NACELLA. 115 



Georgia and Kerguelen islands, by the eastward sweeping Antarctic 

 current, carrying them upon sea weeds. 



A number of West American species placed under Nacella by 

 Carpenter and others belong to Acmcea. 



Two sections, having but slight distinctive characters in either 

 shell or soft parts, have been instituted : 



(1). Nacella s. sir., having the gills smaller in front, the shell thin, 

 light, nearly smooth, the apex far forward. 



"(2). Patinella Dall, in which the gills are equal all around, the 

 shell more solid, and deeply colored. 



Section Nacella Schum. (restricted.) 



The dentition has been figured by Dall, and lately with more 

 exactness by Thiele. 



According to Thiele, the typical N. mytilina is more closely allied 

 in dentition to Patinella than to the other species of Nacella 

 recognized by him, N. vitrea and N. hyalina. The rhachidian tooth 

 is rudimentary and bears no cusp. The inner side-tooth (homolo- 

 gous, according to Thtele, with the second lateral in Patella) is 

 broad, truncated, and has an outer cusp ; the second side tooth is 

 posterior, and bears a small cusp on each side of the broad, truncated 

 median cutting edge. PL 74, fig. 3, represents N. vitrea Phil. ; pi. 

 74, fig. 4, represents N. mytilina Helbl. 



Only one species of this section, as restricted, is known. It lives 

 upon the great sea weeds of the Tierra del Fuego shores. 



N. MYTILINA Helbling. PI. 50, figs. 32-39. 



Shell thin or fragile, elliptical, convex, the apex strongly curved 

 forward and downward near the anterior end ; surface smooth except 

 for narrow, faint, separated radiating riblets, more distinctly devel- 

 oped in front. Color usually light brownish, the apex coppery. 



Inside silvery, large specimens usually having a coppery stain in 

 the middle. Length 41, breadth 24, alt. 17 mill. 



Sis. of Magellan ; Kerguelen Is. 



P. mytilina HELBLING, Abhandl. einer Privatgesellsch. in 

 Bohmen zur Aufnahme der Mathematik, der vaterlandischen 

 Geschichte u. der Naturgeschichte, iv, p. 104, t. 1, f. 5, 6, (1779). 

 P. mytilina GM., Syst. xiii, p. 3698. SCHUB. & WAGN., Conch: 

 Cab. xii, p. 124, t. 229, f. 4052, 4053. P. conchacea GM., 1. c., p. 



