SPA'l'IIA I313 



Family MUTELID.-^. 



Shell usually without sculpture throughout; beaks smooth 

 or but faintly corrugated, never exhibiting the remains of an 

 embryonic shell ; with or without teeth, which, when present, 

 are irregularly taxodont, the shell showing vestiges of them in 

 all the genera ; nacre soft, richly tinted, generally surrounded 

 with a wide, prismatic border ; escutcheon large, distinct ; pal- 

 lial line usually simple, but sometimes having a slight sinus 

 posteriorly. 



Animal having the labial palpi large, rounded below, gen- 

 erally without free points, scarcely or not at all united poste- 

 riorly ; anal and superanal openings not separated ; mantle gen- 

 erally closed behind into branchial and anal siphons, which are 

 alw^ays separated by a strong bridge ; marsupium occupying 

 the inner gills ; embryo a lasidium, composed of three seg- 

 ments, the anterior head-like, the median bearing a single shell, 

 the posterior tail-like. 



Genus S PATH A Lea, 1838. 



Spatlia Lka, Tr. Am. Phil. Soc, \"1, 1838, p. 141, footnote. 



Spathella Boukc.uignat, Esp. Ouk., 1885, p. 13. 



Leptospatha Rochubrune and Germain, Mem. Soc. Zool. Fr., 



XVII, 1904, p. 25. 



Shell elliptical ; beaks very slightly or not at all sculptured ; 

 left valve having a faint, compressed tooth under and in front 

 of the beak, which fits into a corresponding depression in the 

 right valve ; the entire hinge area often covered with longi- 

 tudinally folded epidermal matter ; escutcheon dark, sharply 

 triangular; beak cavities shallow, with a single, elongate, deep 

 scar in each ; two large anterior muscle scars and two posterior 

 ones, the upper small. 



Animal having the palpi longer than wide, semicircular, at- 

 tached along the upper edge, not united ; mantle margin united 

 behind so that the branchial and anal apertures are closed ; on 

 the under margin the mantle is entirely open ; outer branchiae 

 united to the mantle to their extremity ; inner the larger, free 



