8o NAIADES OF MISSOURI 



cavities; hinge teeth usually flattened, very wide and heavy; 

 nacre bright pink; epidermis leathery-red with rays on anterior 

 part of shell ... 



Miscellaneous Remarks: — This form of catillus is the most 

 common of the Pleurobemae in the Osage-Gasconade system and 

 is not found any where else in the state. This " solida" variety 

 is characteristic for its small, solid, subglobular shell, with a 

 more rounded posterior end and less compression for the posterior 

 half than in typical catillus. The radial furrow of the type species 

 (obliquum) is entirely lost. The only difference between this form 

 in the two basins (that of the Osage and Gasconade) is in that of 

 nacre-color — the Osage never varying from pink and the Gasconade 

 shell always with white-nacre. The almost endless inter-grada- 

 tions for ohliquiini and caiillns seem to be the general rule rather 

 than exception, but here m Central Missouri the variety herein 

 described is predominant. Another form occasionally met with in 

 the Osage and grading in between this subspecies and catillus is 

 one that comes near to jiilgidus of Lea, but it would not be listed 

 on account of its rare occurence and doubt whether it should be 

 really separated from this variety, obliqmim catillus. Hundreds 

 of individuals of this form were examined daily throughout the 

 entire month of July, when it was in the height of its breeding season, 

 to find it only gravid (without exception) in its outer gills. It is 

 found to be tachytictic. Perhaps the best idea can be otbained, 

 concerning the difference of the Central Missouri catillus from 

 that of South Missouri, by comparing two average measurements 

 of mature shells given in the following: 



Sex Length Height Diameter Locality 



c? 87 X 64 X 41 (White R., Hollister)— S. Mo. 



cf 54 X 44 X 30 (Osage R., Warsaw) — Cen. Mo. 



In both the radial furrow of typical catillus is effaced. The 

 measurement of the latter removes it so far from the type species 

 that probably a good species might be made out of it. 



Pleurobema obliquum coccineum (Conrad). 



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

 PL XX, Figs. 61 A, B,C and D. 



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

 1839 — Pleurobema obliqmim coccineum (Conrad) Ortmann, Pr. Am. 

 Am. Phil. Soc., LII, pp. 287 — 390. 



