88 NAIADES OF MISSOURI 



ventrad, inner wider but not much longer, inner laminae almost 

 entirely free from visceral mass; palpi medium size; color of soft 

 parts whitish suffused with black; only outer gills marsupial; 

 glochidia small, suboval, spineless. 



Shell Characters: — Shell thick, heavy, subsolid, rhomboid- 

 ovate, longitudinal axis straight, disk smooth, beaks rather low, 

 not near anterior end, sculptured with a few fine concentric ridges 

 angled at the base of the post-umbonal ridge; epidermis brown to 

 black, faintly rayed or rayless; hinge teeth heavy; nacre varying 

 from white to deep purple and violet. 



Miscellaneous Remarks: — This genus falls into two groups 

 for this State. The first group is represented by E. nigra (Raf.) 

 which possesses a heavy subquadrate of subtrapzoidal t3^pe of 

 shell but with obscure beak sculpture; the second group is repre- 

 sented by E. dilataia (Raf.) which has more of an elongate, gibbosed 

 shell with a beak sculpture of thick, heavy, ridges running parallel 

 to the growth lines. The two other groups of this genus (that is, 

 headleianus and complanatus groups) are not found in Missouri, 

 the former being mostly a representative of some gulf states and 

 the latter of the immediate Atlantic drainage. Dr. Ortmann 

 used " Elliptio" as a generic name available for the "American 

 Unio" and employs the original name, " Unio," in the sense of 

 the "European Unio". The soft parts of this genus being practi- 

 cally indentical with those of the genera immediately preceding, 

 the species are indicated entirely on the basis of peculiar shell 

 characters. 



Elliptio nigra Rafinesque.' 

 ("Elephant's Ear," "Pink.") 

 PL XXI, Figs. 64 and 6§ A and B. 

 1820 — Unio (Elliptio) nigra Rafinesque, Ann. Gen. Sci. Brux., V, 



p. 291, pi. IvXXX, figs. I — 4. 

 1823 — Unio cuneatus Barnes, Am. Jl. Sci., VI, p. 263. 



ANIMAL CHARACTERS. 



Nutritive Structures: — Branchial opening large, well set 



' Simpson (1900 b, p. 706) applied the name " U7tio crassidens Lamarck" 

 to E. nigra Raf., but as previously stated under the description of Mega- 

 lonaias (IV, p. 124) that close student of Lamarckian types, Mr. Bryant 

 Walker, has settled the question by pronouncing U. crassidens (18 19) as 

 the so-called U. trapezoides of Lea (1831). 



