THE NAIADEvS OF MISSOURI l6l 



and very much so ventrad from original edge of sterile marsupium, 

 ventral tips of ovisacs teat-like, not colored; conglutinates white, 

 discharged in broken disintegrated masses; glochidium 'ax-head 

 shape or celtiform, small with spine-like structures measuring 

 o.ioox 0.155mm; branchial mantle edge thickened, lamellar, but 

 without palpilae. 



SHELL CHARACTERS. 



External Structures: — Shell elliptical compressed, thin, 

 bialated, post-ala being drawn near to the beaks in definite lobes 

 of growth, sometimes curved laterally; disk without any sculpture: 

 post-umbonal ridge absent, female shell swollen post- ventrad ■ 

 epidermis brown-glistening horn-color with faint rays and areas 

 of indigo blue especially on post wing; beaks low, suppressed 

 sculptured with a few fine concentric lines and a row of three small 

 tubercles on line with a post-ridge. 



Internal Structures: — Cardinals thin, erect,, double in 

 right, single in left valve, laterals rather reduced; scars somewhat 

 faintly impressed; beak and branchial cavities rather shallow; 



Locality 

 (102 R., St. Joseph) 

 (Mud Lake, Kenmoor) 

 (Platte, R., Agency Ford) 

 (Lake Contrary, St. Joseph) 



The last measurement of a juvenile — the youngest and smallest 

 Naiad shell ever found gravid by the writer. Its glochidia were 

 normal. Many of these juveniles are in the writer's cabinet, 

 having been collected in "nests" from L. Contrary for the most 

 part. The shells are like those of ground glass in color and trans- 

 luscent both externally and internally. Beaks are rather apicu- 

 lated and marked by rather coarse concentric ridges with three 

 teat-like tubercles arranged in a row on line with post-ridge, resemb- 

 ling juvenile beak sculpture of Lasmonos. 



Miscellaneous Remarks: — This species may represent the 

 critical transition period from the primitive to the actual modern 

 forms. Its glochidium is not a true Proptera/orm w wo^ possessing 

 typical spines at the ventral corners of its valves. Coker and 

 Surber (1911, pp. 179-182) have pointed out its metamorphosis in 

 the parasitic life as eccentric in that the glochidium remains 



