NATURAL SCIENCES OF PHILADELPHIA. 591 



suggest that the mollusc now under consideration may properly be set apart 

 under a separate generic designation between Truncatella and Melania if it be 

 not retained in Melania. 



It certainly cannot be retained in Amnicola, as its external anatomy is too 

 unlike that of Amnicola in any respect to sanction such treatment. 



Inhabiting the waters in various parts of the United States is a small mol- 

 lusc first noticed by Mr. Say under the name "Melania isagona." Since then 

 it has been assigned by different writers to Anculosa, Paludina and Amnicola. 

 To Mr. Lea is assigned the credit of first surmising the generic place of this 

 mollusc, and an examination of the soft parts of this mollusc in connection 

 with other species of Amnicola confirms the correctness of Mr. Lea's decision 

 in placing this mollusc with Amnicola. The most marked feature observed in 

 the soft parts of this species when compared with other species of Amnicola, is 

 the shortness and width of the foot. In this respect the foot corresponds 

 with the shell. It may be well to remark here, that the length and breadth 

 of the foot in all species of Amnicola observed by the writer, correspond to the 

 length and breadth of the shell. 



The following notes on Paludina have, in part, been some time prepared. 

 Their publication at this time may serve a useful purpose. 



Paludina Integra, Say, (De Kay.) Soft parts. 



The coloring of the external soft parts presents some variety, but less than 

 has been observed in other univalves found in the Mohawk River and Erie 

 (Janal. A characteristic specimen exhibits the following features : 



Upper surface of foot mottled with numerous coalescing orange spots, more 

 or less brilliant, the interstices being slate color with a faint purple tinge, in 

 a strong light almost black, by contrast with the orange spots. The longer 

 diameters of these spots are directed toward the anterior central part of the 

 foot, causing them to appear somewhat as if radiating from that point. Be- 

 neath the shell (on the neck) and anterior to the opercle the surface is nearly 

 destitute of spots, and of a light slate color. On the inferior surface of the 

 foot, the spots are less numerous, smaller and diminishing toward the central 

 line, on each side of which is an undefined space nearly free from spots. 



Removed from the shell, the mantle is observed to be densely covered with 

 pigmentum nigrum, from the margin bordering the aperture of the shell to very 

 near the last apicial whorl. The parts of the mantle which line the outer 

 portions of the shell are more densely covered with pigment than the part em- 

 bracing the columella. 



Embryonic specimens when ready for exclusion are nearly 3-16th inch long, 

 the shell translucent, pale olivaceous and permitting two or three faint black 

 linear bands on the mantle to be seen through it. These black lines subse- 

 quently extend, and form the coating of pigment observed on the mantle of the 

 adult. This species, as well as other species of Paludina of this country, (and 

 perhaps this may be observable in all viviparous univalve molluscs, as it 

 certainly is to a greater extent in a genus of air-breathing molluscs of the 

 Sandwich Islands,) affords about two per cent, of reversed specimens among 

 its young. A considerable number of specimens of this species having this 

 character have been found full grown, bearing young. The young in such 

 instances show only a slightly increased percentage of reversed specimens. 



It is not unusual to find among the embryonic young of this species, speci- 

 mens in which the whorls are made to embrace each other, as in Planorbin. 



Specimens have also been observed in which the whorls were lax and sepa- 

 rated from each other. But no living specimens separated from the parent 

 have ever been detected with these features. 



Other specimens have been observed in which the whorls are drawn nearer 

 the axis of the shell, making the shell elongated. In these specimens the 

 parts which have been drawn away from the preceding whorls are eroded and 



1862.] 



