i: 



174 THE NAIADES OF MISSOURI 



The peculiarity of the shell of this sub-species (as well as that 

 of the species) is that the younger the shell the very much thinner. 

 It is also more brilliantly tawny and green rayed. 



Miscellaneous Remarks: — vSurely this form of brevicula, 

 found by the writer in its type locality, Niangua River, has enough 

 peculiar characters to entitle it to a good species, as Simpson had 

 first considered it. Its special characteristic is the very w4de, deep 

 emargination in the female shell at its post-ventral margin. How- 

 ever, it is almost identical with its parent species as to its soft parts. 

 Its tentacled lamellar-like flap on the mantle edge antero-ventrad 

 to the branchial opening is somewhat like that of ventricosa and 

 hence might be grouped under the Lamp, luteola group ; however, 

 the smaller papillae along the posterior end of the flap (or rather 

 thickened mantle edge) would class it more as an Eurynia. Brittsi 

 is to be distinguished from its parent by the greater post-ventral 

 sulcus (Fig. 99), which extends in as a rather deep radial furrow 

 for a short distance forming the greatest inflation of the shell in 

 front of this. It also differs in shell characters from the female 

 species {brevicula) in not being so broad posteriorly and not as 

 rounded post-dorsad. Dr. Britts collected the originals from 

 Niangua River and sent them to Simpson for naming; hence the 

 consequent name of this species. Cotypes (now in the hands of 

 the writer and illustrated herewith) collected from almost the same 

 point in the Niangua show a decided difference from cotypes of 

 Call's brevicula many of which are now in the writer's collection, 

 through the kindness of Mr. B. F. Bush, one of the most active 

 students and collectors of Naiad shells now living in this State. 

 This sub-species is bradytictic as inferred from the waiter's brief 

 breeding record. He has had the good fortune to collect the glo- 

 chidia oj this form for the first time. In all probability this glochi- 

 dium is the same as that of Call's brevicula. However, its breeding 

 season seems to be a little difl"erent as the writer collected many 

 of the species only a day or two later to find them all sterile. 



Sub-Genus Eurynia (Sens. Strict.) Rafinesque. 



1912b — Eurynia (as sub-genus), Ortmann, An. Car. Mus., VIII, p. 338. 

 (Type, Unio recta Lamarck.) 



Animal Characters: — Differs from those of Micromya in 

 the structure of its rough mantle edge antero-ventrad to branchial 

 opening being more differentiated into a greater number and longer 



