THE NAIADES OF MISSOURI 387 
THE NAIADES OF MISSOURI.—VI. 
BY WILLIAM I. UTTERBACK. 
Lasmonos simpsoni (Ferriss). 
(Simpson’s Shell.) 
PE XXXVI; Figs. (or ‘A: and BP. 
1900—Lampsilis simpsont Ferriss, Nautilus, xiv, p. 38. | 
ANIMAL CHARACTERS unknown to writer. 
; SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell subovate, elongated, sub- 
solid, narrowly rounded in front, broadly rounded behind, some- 
what alated; disk smooth; beaks low, sculptured by ridges some- 
what corrugated and nodulous; epidermis tawny with green rays 
with rather roughened growth lines; hinge line evenly curved. 
INTERNAL STRUCTURES:—Cardinals reduced to mere stumpy 
rudiments; laterals single in right, very faintly double in left 
valve; anterior scars rather deep, posterior shallow; nacre bluish 
shaded with violet, copper or pink. 
Sex Length Height Diameter Locality 
oS 78 x 46 x 28mm. (White R., Hollister) 
Q 95 x 50 x 22 “é ( «6 “e “6 ) 
MISCELLANEOUS REMARKS:—No juveniles, nor young shells, 
have been obtained by the writer in this State. However, Mr. 
Ferriss, the author, has obtained a good suite from Spring River, 
Hardy, Arkansas, and has described the young shells as possessing 
an epidermis with green rays and ‘“‘coppery beaks,”’ a well marked 
post-umbonal ridge and low but distinct beaks with coarse corru- 
gations. The author of this species, groups it between Lasmonos 
fragilis and leptodon. From the former it may be distinguished 
by a thicker more elongated shell, by more reduced hinge teeth 
and less varigated nacre and from the latter by being more inflated, 
wider, thicker, not so pointed posteriorly and with fewer rays. 
Hence its place can be fairly well determined conchcologically. 
Future studies of its marsupial and glochidial characters of this 
species, as well as that of leptodon (=S. teniussimus Lea), may 
classify it far differently, since shell characters are too liable for 
parallelism and convergence as true bases of classification. 
