112 



epidermis over the umbones is very light or straw-yellow in color; the ilorsal 

 aspect of the posterior umbonal slope is characterized by the presence of two 

 rather indistinct and obtuse angles which extend from the ambones and, reaching 

 the posterior margin, form the characteristic biangiilation seen in themah-; 

 cardinal teeth short, acuminate, single in the right and double in the left valve, 

 the single tooth being flattened and plate-like, the doable tooth somewhat more 

 trigonal and heavier, all crenulated on the margins; the posterior teetii are long, 

 slightly curved, and lamellar; plate between the cardinal and ]>osterior teetli 

 scarcely evident; the anterior adductor cicatrices are large, and deeply impressed, 

 entirely distinct from that of the protractor pedis impression which i'^ deep and 

 often pit-like; the posterior cicatrices are confluent, scarcely evident, that of 

 the retractor pedis muscle being placed at extreme end of the posterior hinge teeth; 

 dorsal cicatrices arranged, usually, in a line of five or more in the shallow cavity 

 of the umbones, though in an occasional specimen they are grouped : the pallial 

 cicatrix is faintly but regularly impre.ssed throughout its entire length; nacre 

 white, with tendency to salmon in the cavity of the umbones. beautifully irides- 

 cent posteriorly. 



The four specimens on which this diagnosis is based are from Lake Caddo, 

 Louisiana. Their dimensions are the following, the first being that of a female; 

 comparison with the remaining three will evidence the more compressed character 

 of the male shell: 



No.l. No. 2. No. 3. No. 4. 



-Length 40.00 mm. 36.50 mm. 39.50 mm. 38.50 mm. 



Height 24.00 mm. 20.00 mm. 22.00 mm. 21.50 mm. 



Breadth .... 18 51 mm. 14.50 mm. 14.50 mm. 13.(j0 mm. 



The habits of this form are quite similar to those of the type of the group. 

 It delights in still water with muddy bottoms, and usually occurs in very great 

 numbers wherever it is found at all. 



As may be seen by comparing the figures given in the plates, which are copies 

 of Lea's original figures, this form illustrates the erection of a species name upon 

 characters that are but an expression of sex. 



"The anatomy of the animal has been considered, rather than authority, in the teruii- 

 nology adopted. Thus the lengtli it' the extreme distance from the anterior to i)Osterii)r mar- 

 gin; the height the distance from ligament to the ventral margin: the width the distance 

 measured by a line drawn through the animal, transversely, from valve to valve. This 

 appears both natural and satisfactory. Say, Kirtland, Barnef^.Sowerby and others with them 

 confused the anterior and posterior ends; Lea did not make this blunder, but made others 

 equally reasonles.-;. Thus the distance from valve to valve he calls the heighlJi, a.-; if the nor- 

 mal or proper position of the animal was on one of its valves. Some later writers appar- 

 cntl.v have such reverence for these blunders that they still employ an obsolete terminology. 



