THE NAIADES OF MISSOURI I5I 



and indistinctly double-looped ridges; post-umbonal ridge square 

 cornered from beaks to post-extreme; epidermis yellowish or light 

 brown painted with rays broken into square or lunate blotches; 

 female shell smaller, thicker, shorter and more inflated than male. 

 Internal Structures: — ^Cardinals rather low, ragged the 

 the right one having four vertical subparallel ridges; interdentum 

 broad; laterals stout, rather long and sharply inclined; beak 

 cavities, moderately deep; nacre silvery white, irridescent. 

 Sex Length Height Diameter Locality 



cf 96 X 79 X 33mm (Merame R., Meramec Highlands) 



9 70 X 56 X 37mm (Marais des Cygnes, Athol) 



9 67 X 53 X 30mm (Miss. R., Hannibal) 



cf 48 X 36 X i6mm ( " " LaGrange) 



Miscellaneous Remarks: — P. securis being more of a 

 marine type of shell than fresh water renders it worthy of the 

 creation of a genus. The young shell is like that of an "exquisite 

 shell from the sea-shore," so narrowly flattened is it post-dorsal 

 portion of shell, so delicately painted with broken lunate rays 

 and so arched forward are its compressed and pointed umbones. 

 This species is not found at all in the interior north of the Missouri 

 River; neither is it found in Southwest Missouri. It is rather 

 common in the Osage, Gasconade and Meramec though not very 

 typical as a rule. The most perfect securis is the Mississippi. 

 The writer found this species from the Mississippi bearing ripe 

 glochidia June 22nd; Wilson and Clark (1914, p. 52) found the 

 Cumberland securis in the same gravidity June 3-16. Dr. Ortmann 

 finding it gravid in late fall again fixes its breeding season as 

 normally bradytictic. 



Genus Lasmonos Rafinesque. 



1820 — Lasmonos Rafinesque, Monograph, Ann. Gen. Sci. Phys. Brux. 

 1911b — Paraptera Ortmann, Mem. Car, Mus., IV pp. 331, 334, 338. 



(Type, Lasmonos Jragilis Rafinesque.) 



Animal Characters: — Siphonal openings large inclined to 

 be tubular; supra-anal high, well separated from anal; inner 

 laminae of inner gills entirely connected to visceral mass; palpi 

 free their whole length post-dorsad, color of soft parts grayish with 

 yellowish papillae on blackened mantle edge or branchial opening; 

 marsupium kidney-shaped, consisting of several ovisacs occupying 

 posterior part of outer gills; conglutinates white, leaf-like, not 



