AND ALLIED PORMS. 



19 



Among: Fraueiifeld's synonyms we find the name of Microno, 

 Ziegler, MSS., which is also equivalent to BythineUa. 



We have among our North American Hydrobiinse, species 

 which should, with little doubt, be referred to the genus Bythi- 

 neUa. I have not, indeed, had opportunities of studying the 

 entire soft parts of any of these species, nor of ascertaining the 

 form of their eggs — having, except in one instance, to rely upon 

 the shell alone for indications of the generic affinity. 

 That instance is the Amnicola Nickliniana (Lea), Halde- Fig. 8. 

 man, an elongated species (Fig. 8), of which I have ^ 

 found in the Smithsonian collection specimens containing (\) 

 the dried animal, and have thus been enabled to study XJ 

 the lingual dentition. This (Fig. 9') I find to be nearly 

 of the same type with Troschel's Amnicola (Subulina) ihermalis, 



Fig. 9. 



the rhachidian tooth having but one distinct basal tooth on each 

 side, though an obtuse lateral lobe is developed below it, and a 

 t-denticulated cusp ; the intermediate tooth has the inferior pro- 

 cess or peduncle longer than the body (which has a central cavity 

 and an infero-interior projection not seen in Troschel's figure of 

 the dentition of thermalis), and a 6-denticulated cusp ; the inner 

 lateral has the outer margin of the shank reflexed or thickened, 

 and has a 12-denticulated cusp; and the summit of the outer 

 lateral is apparently smooth, although it has once appeared to 

 me serrated, under a very high power ; if so, the denticles must 

 be exceedingly minute and numerous. 



In view of the shape and obtuse apex of the shell, and the 

 character of the dentition, I do not hesitate to place the Palu- 



' It should be remarked that in this figure, as in most other figures of 

 lingual dentition in this paper, the teeth are represented thrown a little 

 out of their natural positions in respect to each other in order to show 

 more distinctly the form of each. 



