ISrAiADES OF MISSOURI 63 



Internal Structures: — Cardinals heavy, ragged, double in 

 left usually single in right; laterals long, rather straight and 

 heavy; beak cavities deep; nacre marble white, occasionly pink, 

 irri descent posteriorly 



Sex Length Width Diameter Locality 



9 169 X 88 X 48 (Grand R., Darlington) 



cT 150 X 78 X 44 (Platte R., Agency Ford) 



cf no X 50 X 30 (Osage R., Warsaw) 



54 X 30 X 15 (White R., Hollister) 



No juveniles obtained. The last measurement is that of the 

 youngest verrucosa obtanied. Its beak sculpture and disk are 

 entirely sculptured with nodules and tubercules. The slope of 

 the post-dorsal ridge are sculptured with three or four coarse, 

 costated undulations and with numerous fine costae arranged 

 dorsad; shell very greatly compressed; nacre bluish. 



■ Miscellaneous Remarks: — Q. verrucosa is the most peculiar 

 species of its genus on account of the sexual dimorphism of its shell. 

 For this reason especially, Simpson created a special genus {Trito- 

 gonia) for it. Some students are inclined to think that Tritogonia 

 deserves sub-generic rank at any rate, because of its morphological 

 departure form the typical Quadrula shell. Its soft parts, however, 

 are so identical with those of the typical Quadrula that there is no 

 reason for its groupings with any other genus. Even though the 

 form of the shell may be differnt, yet its conchological parts corre- 

 late with those of other Quadrulae. Although Rafinesque's figure 

 of this species is abomnable, yet an unbiased study of it, together 

 with that of his good description, would give preference for the 

 adoption of his verrucosa over that of Barnes' tuberculata. Like 

 Q. quadnda, its breeding season is about as eccentric, in that the 

 mature glochidia are not retained in the marsupia for, any length 

 of time; hence this accounts for the great difficulty of securing its 

 larvae for study. Surber was fortunate in securing specimens 

 with ripe glochidia June loth. The writer would judge from this 

 record and that of his own (i. e., sterility for the Fall and Winter 

 months) that this species is tachytictic. Vercucosa has the widest 

 distribution for this State: however, it varies somewhat in size, 

 inflation, disk sculpture, nacre-color for the different sections of 

 the vState; e. g., the pink-nacred ones are exclusively confined to 

 the vSouthern Missouri streams. 



