50 Amkrican IvAST Coast Arcas 59 



Area incongrua S;iy 

 Pl;iU' XI\', Fi,i;ures 4, 5, 6, 7 



^liiii iiiiOiignui Say, Acacl. Nat. Sci. I'hila., Journ., vol. 2, p. 268, 1.S22. 

 Ana incongrua Reeve, Conch. Icon., Area no. 50, 1844. 



Scap/iarca {Cuiiearca) incongrua Dall, Wagner Free Inst. Sci., Trans., vol. 3, pi. 4, pp. 618, 635, 

 659, 189S. 



"Shell somewhat rhomboidal, with from twenty-six to twenty-eight ribs, placed 

 nearer to each other than the length of their own diameters, and crossed by elevated, 

 obtuse, equal and equidistant lines, which are altogether wanting on ten rays of the disc 

 of the left valve; apices opposite to the middle of the hinge, distant from each other, 

 with a lanceolate space between them, of which the breadth is about one-third of its 

 length; extremities of the hinge margin angulated; posterior edge rounded; inferior edge 

 rounded, that of the left valve extended a Httle beyond the regular curve in the middle; 

 anterior margin cordate, flattened; anterior edge nearly rectiUnear * * *. 



"This species, which is very abundant on our coast, strongly resembles A. rlwmbea, 

 but, agreeably to the figure in the Encyc. Meth., it differs in the width of the space on 

 the hinge margin, in the width of the spaces between the ribs, and in its more rectilinear 

 anterior edge." — Say, 1822. 



In the original description right and left, anterior and posterior are interchanged. 



Left valve much the larger; ribs twenty-six to thirty; left valve with all the ribs 

 wider than tlie interspaces, which are very narrow about the center of the shell; anterior 

 ribs coarse, posterior finer; interspaces widest on the anterior part; ribs with transverse, 

 conspicuous, raised lines which are long and narrow except near the umbo, where the\' 

 are rounder; ribs of the right valve from the center to the umbonal ridge narrow and 

 nearly smooth ; left valve with a shallow sulcus anterior and parallel to the umbonal 

 ridge; ligament covering practically all the cardinal area and bounded by a deep groove; 

 ligament area with faint transverse striations; teeth longer and oblique distally. A. in- 

 congrtca shows hardly a trace of an interstitial rib on the right valve. There is a thin 

 light brown epidermis. 



This is one of the commonest east coast shells. The closely related form A. hra.ul- 

 iana Lamarck; Plate XIV, Figure 8; (An. s. Vert., vol. 6, p. 44, 1819), is smaller than 

 A. iiifouoriia, it is proportionately shorter and higher with a short, abrupt posterior slope 

 and straight posterior margin. The abruj^t change from nodulous to smooth ribs on the 

 right valve produces an apparent sulcus on the side of the umbo. The shell from the 

 West Coast figured by Reeve as A. Brasiliatia is referred by Dall to A. nodosa Wood. 

 Dall says, (Wagner Free Inst Sci., Trans , vol. 3, p. 635), "The typical A. inamgrna is 

 quite variable in form, and I have not seen specimens which could be unhesitatingly re- 

 ferred to it from older beds than the Pleistocene, or more southern localities, living, than 

 the coast of Texas. Here it is mixed with specimens of the braziliana type, towards 

 which the incongrua tends to vary. The Costa Rica Pliocene fossils are exactly like bra- 

 ziliana: the Antillean shells also, while van,'ing a good deal, retain the dimensions of bra- 

 siliava and more or less of its other characters. It is probable th.at the two forms would 



