THE NAIADES OF MISSOURI . I05 



when charged, ventral edges distended, water canals present, 

 no specialized structure of mantle edge antero-ventrad to branchial 

 opening; glochidium large, broadly spadiform, spined, hinge 

 line straight. 



Shell Characters: — -vShell thin, subalated, smooth on disk; 

 beaks flat, sculptured with four or five rather double-looped ridges ; 

 epidermis smooth, polished, rayed in green in the region of the 

 post-umbonal ridge; hinge teeth absent; scars faint, confluent; 

 nacre bluish. 



In this state this genus is represented by the two species, 

 Las. ohiensis (Raf.) and suborbiciilaia (Say) — -the latter not having 

 been completely described hitherto. The author has had conve- 

 nient access to large beds of suborbiculata and has been fortunate 

 in securing specimens gravid with embryos in all stages and with 

 mature glochidia. Neither has the latter been figured nor des- 

 cribed before. Because of the fact that the marsupium of subor- 

 biculata is more like that of Arcidens and that of ohiensis closer to 

 Anodonia we would group the latter as more modern; then, too, 

 the hermaphroditism and longer breeding season of ohiensis 

 would also indicate an advance in being able to perpetuate the 

 race. 



Lastena suborbiculata (Say). 



("Suborb," " Heel-splitter.") 



PL IV, Fig. 19a; PI. IX, Fig. 19; PL XXIII, Figs. 73 A~D. 



1 83 1 — Anodonia suborbiculata Say, New Harm. Diss. (Newspaper 

 form); Am. Conch. I, No. II, 1831 (Later date), p. XI. 



1867 — Anodon suborbiculatus Sowerby, Conch. Icon., XVII, PL V, fig. 1 1, 

 II. 



ANIMAL CHARACTERS. 



Nutritive Structures: — Branchial opening comparatively 

 small, upward curved with many fine orange colored papillae; 

 anal also directed upward, smooth with Y-shaped yellow markings; 

 suora-anal long, far removed from anal by mantle connections; 

 inner gills wider but very little longer than outer, inner laminae 

 of inner gills not connected to visceral mass; palpi rather long, 

 united antero-dorsad about one-third of their length; pericardinal 

 region very large, watery, pinkish-brown in color; foot, long, 

 thin, deep orange in color, adductors also orange, yellowish re- 

 tractors and protractors visible through the watery, transparent 



