FRESH WATER AND LAND SHELLS. 293 



by D'Orbigny. It is very peculiar in the texture of the epidermis, which is crimpled up 

 into irregular, somewhat parallel, very close, rugose lines, which cover the whole surface 

 of the disk. In both of these valves there is a distinct muscular impression above the 

 great anterior cicatrix, which I have not observed before in any of the Anodonise. The 

 absence of a dorsal cicatrix is also unusual; but in this it resembles some of the other 

 South American species, viz.: Anodonta ensiformis, Spix, soleniformis, D'Orb, &c. In 

 outline it is nearly allied to An. cxi/is, (Nobis,) but it has not the straight dorsal line, 

 is a thicker shell, and has a very different epidermis. The triangular fosset at the end 

 of the ligament is rather large and well marked. The right valve has a few black pen- 

 cilled marks in the nacre, such as are sometimes seen in the South American Anodonise. 

 These are remarkable as being nearly parallel, and in a direction pointing to the beaks. 

 They look as if made by a pen, with black ink. 



I name this fine species in honour of tho memory of Schroster, author of Flusconchlien, 

 &c. 



Anodonta Arkansensis. PI. XXIX. Fig. 56. 



Testa Iwvi, ellipticd, compressd, valde insequilaterali, postice subbiangulatd ; valoulis crassis; natibus 

 subprominentibus, ad apices undulalis ; epidermide subviridi, radiald, striata; margarild csetuleo-albd. 



Shell smooth, elliptical, compressed, very inequilateral, subbiangular behind; valves thick; beaks rather 

 prominent and undulated at the tip; epidermis greenish, rayed; nacre bluish-white. 



Ilab. Little Arkansas, J. L. Le Conte, M. D. 

 Cabinet of Major Le Conte. 

 Diam. .0, Length 1.7, Breadth 3 inches. 



Shell smooth, elliptical, compressed, subbiangular behind and rounded before, sub- 

 angular on the posterior slope; substance of the shell thick; beaks rather prominent, 

 pointed, with three or four large undulations at the tip; ligament long and rather thin; 

 lermis greenish, striate, with unequal transverse marks of growth, dark on the pos- 

 terior slope, with indistinct, broad greenish rays over the disk; posterior slope with two 

 lines from the beak to the posterior margin; dorsal margin slightly curved, and having a 

 callus immediately under the beak almost amounting to a tooth; anterior cicatrices dis- 

 tinct; posterior cicatrices confluent; dorsal cicatrices placed on the lower part of the 

 callus; cavity of the shell rather shallow; cavity of the beaks shallow and angular; nacre 

 bluish-white, leaving a broad dark margin. 



Remarks. — A single specimen of this interesting species has been submitted to m.> by 

 Dr. J. L. Le Conte, who obtained it on a late scientific excursion to the Rocky Monti- 

 tains, from the Little Arkansas, where the road to Santa IV crosses it. This species is 

 not likely to be confounded with any other known to me, and belongs to that group which 

 has an irregular thickening or callus under the beak on the dorsal margin, which, while 

 it does not amount to a tooth, makes a very distinct natural group. This callus is not 

 so large and marked as to be confounded with D'Orbigny's genus Monocondylsea ; but it 

 may be compared with that of Alasmodonta edentula, Say, which is a true Anodonta. The 

 form of this species is more lenticular than in any other of the genus I know; and look- 

 k, — 5 1 



