8 RESEARCHES UPON THE HYDROBIIN-ai 



only to those parts of the auirnal which are protruded from the 

 §hell in progression. 



Haldemau, in his " Monograph of the genus Amnicola,^'' which 

 forms part of his beautiful work on the fresh-water gasteropods 

 of North America, also gives a description of the animal, in which 

 he adds nothing of importance to that of Dr. Gould, except short 

 accounts of the gills and of the character of the ova, which do 

 not accord with my own observations as detailed below. 



Dr. Lewis, in the "Proceedings of the Boston Society of Natural 

 History," Yol. YIII, 1861, p. 255, gives a description of the so- 

 called Amnicola lapidaria, stating that the soft parts of this 

 species are " identical in form with Melania,''^ and subsequently, 

 in the "Proceedings of the Academy of Natural Sciences of 

 Philadelphia" for 1862, p. 590, gives a more detailed account of 

 the animal, and points out certain resemblances to Melania and 

 TruncateUa. But, as has been elsewhere noticed,* its resem- 

 blance to the Melanians is only a superficial one, and it is far 

 removed from that group in the structure of its generative 

 organs. To the Truncatellte the species indeed shows a strong 

 likeness in form and habits, which Dr. Lewis W'as the first to 

 detect, although Say had indeed placed it in Cyclostoma. But 

 its respiratory organs are of a different type, " Amnicola''' lapi- 

 daria being a true Ctenobranchiate, while the Truncatellse, as 

 far as known, are air-breathing moUusks. 



In a paper published in the " Proceedings of the Academy of 

 Natural Sciences of Philadelphia" for September, 1862, Mr. Tryon 

 has elevated the group Amnicola? to the rank of a family, under 

 the name of Amnicolidae, but as this author has given no diagno- 

 sis of the group thus proposed, we are ignorant of the grounds 

 upon which he considered it distinct from the allied families already 

 known and named. He mentions but a single genus, Amnicola, 

 but proposes under it a subgenus, Pomatiopsis, for the elongated 

 species, with A. lapidaria for an example. This species, how- 

 ever, is not congeneric with the other elongated forms ; it being 

 found upon examination to present structural peculiarities which 

 separate it widely from all of the true Amnicolae. 



The suljiject has since been investigated by my friend Prof. 

 Theodore Gill, of the Smithsonian Listitution, whose views are 



' Am. Journ. of Science and Arts, [2] xxxviii (1864) 50. 



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