NAIADES OF MISSOURI 83 



the solidus of Lea and (in the conventional sense) may be called 

 Pleu. solidum. The latter being antedated, we must use the 

 former name. This species is rather typically represented in the 

 Mississippi for Missouri, the mountain streams of South Missouri 

 and also in the Neosho basin of South West Missouri. No real 

 types are ever found in Central Missouri and not even forms are 

 found in North Missouri. This species is distinguished from 

 Pleu. coccinemn by always being rather swollen and having a 

 radial furrow more or less expressed. In this latter character 

 it never reaches the extreme, as seen in obliquum, of the pinched 

 radial groove terminating in sulcus at the post-ventral margin. 

 Its breeding record show^s it to be tachytictic. 



Pleurobema coccineum (Conrad). 



("Flat Nigger-Head," "Pink Pig-toe," "Round Pig-toe.") 

 PL III, Fig. 60b; PL XX, Fig. 60 A and B. 



1836 — Unio coccineus Conrad, Monog., Ill, p. 29, pi. XIII, fig. I. 

 1900b — Quadrula coccinea Simpson, Proc. U. S. Nat. Mu^., XXII, 



pp. 788-789. 

 1912b — Pleurobema coccineum (Conrad) Ortmann, An. Car. Mus., 



VIII, p. 263. 



ANIMAL CHARACTERS. 



Nutritive Structures: — Branchial opening densely set 

 with papillae; anal with "indistinct crenulations and papillae; 

 mantle connection between anal and supra-anal very close; gills 

 wide, the inner wider and longer, inner laminae free from visceral 

 mass; palpi long, united two-thirds of their length; color of soft 

 parts tannish, mantle edge at siphonal openings black. 



Reproductive Structures: — Only outer gills marsupial; 

 when gravid marsupia swell moderately lengthwise in the center; 

 conglutinates white, thin, leaf-shape, discharged whole; glochidia, 

 suboval, spineless, medium in size measure 0.150 x 0.150mm. 



shell characters 



External Structures: — Shell subelliptical to subquadrate 

 varying with age, flat, very much compressed through post half, 

 greatly inflated (comparatively) anteriorly, dorsal and ventral 

 margins curved about the same; beaks rather full not very well 

 placed to the front, sculptured by concentric lines with two or 



