NAIADES OF MISSOURI 55 



Missouri approach more the t3'pical quadrula in being more com- 

 pressed and more tuberculated. Perhaps the most typical quadrula 

 of Missouri is to be found in the geographic center of the state 

 where the flatter, thinner and more "lachrymosed" shell occurs. 

 As in many of the species of these related genera the intergrades 

 are so numerous that we can consider only the most striking ones 

 that may be traced to mere local conditions. The author has 

 found this species to be the most sensitive to discharge its conglu- 

 tinates immaturely when disturbed from its natural bed and then, 

 too, since conglutinates, spawned in nature, have been examined 

 to find them containing late embryos it is to be inferred that 

 maturity may take place outside. Hence we may account for 

 our diflficulty in securing the mature glochidia from the ovisacs 

 of the mother. The writer has examined hundreds of gravid 

 quadrula in mid- and late summer only to find every stage of 

 embryonic development except the glochidial. In this respect 

 this species resembles 0. verrucosa and the fact that the ovisacs 

 of each contain unusually large quantities of mucus may have 

 some association with their eccentric breeding habits. The breed- 

 ing season of Q. quadrula is from May to August and hence is 

 tacytictic. 



Special attention is given here to a deeply sulcated Jorni oj this 

 species which occurs rather commonly in our North West Missouri 

 lakes but which may only be a pathologic condition due perhaps, 

 to parasitic attacks upon the mantle glands that build up the 

 shell. Three type shells measure as follows: 



Sex Length Height Diameter Um. ra. Locality 



cf 85 X 70 X 57mm 0.485 (Lake Contrary, St. Joseph, Mo.) 



9 80 X 55 X 54 " 0.450 ( " " " ■" " ) 



o" 48 X 38 X 40 " 0.435 ( " " " " " ) 



These measurements show an unusual inflation and extra- 

 ordinary position of the umbones. If this should be a normal 

 form it would deserve specific consideration because of the deep 

 post-dorsal sulcation and also because of the wide, deep radial 

 fiurow in front of the prominently angled post-umbonal ridge. 

 However, since Mr. Frierson concurs with the author in the belief 

 that it may only be "a strange freak" after all it would be dismissed 

 here with only the reference to its photograph {PL XVIII, Figs. 

 46 A and B.) 



