112 



epidermis over tlie uiubones is very light or straw-yellow in folor; the dorsal 

 aspect of the posterior umboual slope is characterized by the presence ot two 

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

 the jiosterior margin, form the characteristic biangulation seen in the male;, 

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

 the single tooth being flattened and j)late-like, the double tooth somewhat more 

 trigonal and heavier, all crenulated on the margins; tiie i>osterior teeth are long, 

 slightly curved, and lamellar; plate between the cardinal and i>osterior teeth 

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

 entirely distinct from that of the protractor pe<lis impression which is 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 Ave or more in the shallow cavity 

 of the umbones, though in an occasional specimen ihey are grouped; the pallial 

 cicatrix is faintly but regularly impressed 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 51mm. 14.50 mm. 14.50 mm. 1.3.(i0 mm. 



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

 It delights in still water witii 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, wliieh are oo pies 

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

 characters that are but an expression of sex. 



'■'The anatom.v of the animal ha.s been considered, rather than authority, in the termi- 

 nology adopted. Thus the length is the extreme distance from the anterior to posterior mar- 

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

 measured by a line drawn through the animal, transversel.v, from valve tn valve. This 

 appears both natural and .satisfactory. Say, Kirtland, Barnes, Sowerliy and others with them 

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

 equally reasonless. Thus the distance from valve to valve he calls the heiyhth. as if the nor- 

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

 entl.v have such reverence for tliese blunders that they still employ an obsolete teriiiiiiology. 



