xo. 1205. .s ,-. \ol'SIS OF THE NAIADES SIMPSON. 895 



I do not know where the last five species are described, and they 

 probably Lave never been published. Wright bas named a Texan 

 species Unio iheringi. 



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; palleal line usually simple, but sometimes having a 

 slight sinus posteriorly. 



Animal having the labial palpi large, rounded below, generally with- 

 out free points, scarcely or not at all united posteriorly; anal and 

 superanal openings not separated; mantle generally closed behind into 

 branchial and anal siphons, which are always separated by a strong 

 bridge; marsupium occupying the inner gills; embryo a lasidium, com- 

 posed of three segments, the anterior head-like, the median bearing 

 a single shell, the posterior tail-like. 



Genus SPATHA Lea, 1838. 



(Type, Anodonta rubenx Lamarck.) 

 Spatha LEA, Tr. Am. Phil. 8oc M VI, 1858, p. 141, footnote. 



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 longitudinally 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, attached 

 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 branchia3 united to the mantle to their 

 extremity; inner the larger, free from the abdominal sack; foot tongue 

 shaped, thick; anal and branchial openings separated by a strong 

 bridge. 8. rubens Lamarck (Troschel). 



Subgenus SPATHA Lea, 1838. 

 (Type, Spatha rumens Lamarck.) 

 Shell solid, having faint concentric beak sculpture. 



(Group of Spatha rubens.) 

 Sculpture of the beaks following the growtli lines; shell solid. 



