UNio. 1(37 



the hinge or the form of the shell ; and more recently Professor 

 Agassiz has proposed several genera, not yet published, founded on 

 the peculiarities of the animal, especially of the mantle and the 

 parts of the gills which carry the embryos. 



Ociiiis UNIO, Retzius. 1788. 



Shell equivalve, inequipartite, multiform ; hinge with a stout, 

 irregular, striated, simple or divided cardinal tooth in each valve, 

 and an elongated, compressed, marginal tooth; gills free from the 

 abdominal sac, their posterior extremity attached to the mantle ; 

 eggs filling the whole extent of the outer gill ; upper siphonal open- 

 ing somewhat fringed. 



Unio complanatus. 



Figs. G8, G9, 70. 



Shell elonjjated ovate, somewhat angular posteriorly, inequipartite ; beaks not 

 much elevated, epidermis dark brown; interior purple or saluion-colored ; hinge- 

 teeth deeply striated, pyramidal. 



Mija complanata, Solander, MSS., Portland Catal. 100. — Dillavyn, Catal i. ^\. 

 Unio purpurevs, Say, Nich. Encyc. (Amer. 1st ed.) iv. 3, fij;. 1 (1816). — Desiiaves, 



Encyc. Meth. Vers., ii. 581, pi. 249, fig. 5. — Barnes, Sillinian's Journ. vi. 2(J4. 

 Unio parpurasrens, Lamarck, An. sans Vert. 2d ed. vi. 53.t. 

 Unio violaceiis, Spengler, in Gncrin's Mag. 26. 

 Unio rarisnJcata, coarctuta, rhomlmla, rarinifera, Georgina, (jlahrata, and sulcidens of L.VM. 



(An. sans Vert, vi.) on the anthority of Lea. 

 Unio flnviatilis, Greex. 

 Unio complanatus, Lea, Naiades, i. 30; Trans. Amor. Phil. Soc. (new series) iii. 416 



(1830) ; vi. 130, not of Deshayes. — De Kay, Nat. Hist. N. Y. 188, pi. 22, fig. 246. 



Shell very varia1)le in form, usually oljlong-ovate, sometimes sub- 

 rhomlioidal or sul>oval, very inequilateral, broadest behind, rather 

 compressed. Beaks about the anterior fourth of the shell, little 

 elevated, always much eroded, and exhibiting numerous layers of 

 greenish epidermal matter ; anterior extremity always regularly 

 rounded ; superior margin, behind the beaks, straight and some- 

 Avhat ascending for one half its length, then, suddenly declining, it 

 forms an indefinite angle ; posterior end pointed, rounded or slightly 

 clipped ; inferior margin regularly curved, or sometimes a little 

 arched at the middle ; an obtuse ridge passes from the beaks to the 

 posterior tip. Surface coarsely wrinkled by the lines of growth, 

 and covered by a dark, tar-colored, or very dark-green epidermis. 



