NO. 1205. SYNOPSIS OF THE NAIADES SIMPSOX. 661 



t PEGIAS FABULA Lea. 



Marflarita ( Margaritana) fabula, LEA, Syn., 1836, p. 46; 1838, p. 28. 

 * Margaritana fabula LEA, Tr. Am. Phil. Soc., VI, 1838, p. 44, pi. xin, fig. 39; * Obs., 

 II, 1838, p. 44, pi. xm, fig. 39. "TROSCHEL, Arch, fur Nat.,V, 1839, Pt. 2, p. 

 238.* KUSTER, Conch. Cab. Unio, 1862, p. 301, pi. c, fig. 3.* B. H. WRIGHT, 

 Check List, 1888.* P.ETEL, Conch. Sam., Ill, 1890, p. 173. 



* Unio fabula HAXLEY, Test. Moll., 1842, p. 213; Biv. .Shells, 1843, p. 213, pi. xxii, 



fig. 45.* CATLOW and REEVE, Conch. Norn., 1845, p. 59.* SOWERBY, Conch. 

 Icon., XVI, 1868, pi. LXXVI, fig. 394. 

 *Margaron (Margaritana) fabnla LEA, Syn., 1852, p. 44; 1870, p. 70. 



* Micromya fabula AGASSIZ, Arch, fiir Nat., 1, 1852, p. 47. 

 *Strophitu8 fabula CONRAD, Pr. Ac. Nat. Sci. Phila., VI, 1853, p. 263. 

 *Baphia fabnla H. and A. ADAMS, Gen. Rec. Moll., II, 1857, p. 499. 



* ^Margaritana curreyana LEA. Pr. Ac. Nat. Sci. Phila., 1, 1840, p. 288 ; *Tr. Am. Phil. 



Soc.,VIII, 1842, p. 223, pi. xvm, fig. 40; *Obs., Ill, 1842, p. 61, pi. xvm, fig. 

 40. * KUSTER, Conch. Cab. Unio, 1862, p. 300, pi. c, fig. 2. *B. H. WRIGHT, 

 Check List, 1888. 



* Micromya curreyana AGASSIZ, Arch, fur Nat. 1, 1852, p. 47. 

 *Margaro (Margaritana) curreyana LEA, Syn., 1852, p. 42; 1870, p. 68. 

 *StropMtit8 curreyana COXRAD, Pr. Ac. N. Sci., Phila., VI, 1853, p. 263. 

 *Baphia cnrreyana H. and A. ADAMS, Gen. Rec. Moll., II, 1857, p. 499. 



* Unio curreyana SOWERBY, Conch. Icon., XVI, 1868, pi. LXIII, fig. 319. 



* Unio cnrreyaniis HANLEY, Biv. Shells, 1856, p. 386, pi. xxiv, fig. 10. 

 ^Margaritana correyana P.ETEL, Conch. Sam., Ill, 1890, p. 173. 



Cumberland aud Tennessee river systems. 



Genus ARCIDENS Simpson, 19OO. 

 (Type, Alasmodonta confragosa Say.) 



Shell subsolid, inflated, subrhomboid, with full, high beaks; beak 

 sculpture very strong, consisting of irregular corrugations which fall into 

 two loops, at the bases of which the ridges are swollen into knobs that 

 continue out in two radiating rows on to the disk of the shell; in front 

 of and behind the beaks are many fine, radial wrinkles, the posterior 

 ones being zigzagged; surface of the shell covered with oblique folds 

 and wrinkles; epidermis dark olive, shining; there are two elongated, 

 compressed psuedocardinals in the left valve, the posterior under the 

 beak, and curved upward, cutting off the hinge plate in the right valve, 

 which has a single, compressed psendocardinal in front; laterals numer- 

 ous, short, blurred; muscle scars irregular; nacre white. Animal with the 

 gills very large, rounded below; inner the larger, free from the abdominal 

 sac; marsupium filling the outer gills, of a peculiar, granular texture; 

 palpi very large, elongated, attached half way to the mantle; mantle 

 with a thick, unspotted border; branchial opening large, papillose; 

 anal opening with rudimentary papillae. 1 



1 In the only gravid specimen I have seen the marsupinm presented a peculiar, 

 striated, granular structure, different from that of any Naiad I know. It was nar- 

 rowed in the middle, and produced into a sort of lobe behind, but, though the spec- 

 imen seemed to be a perfectly normal one, I am in doubt whether this peculiar form 

 is constant. I do not think that this granulation is the result of a diseased condi- 

 tion, though it may be. 



