12 



booal plates as expressing specific value only, but his theory has since found 

 no supporters. I have closely re-examined all the Pholades in my collection, 

 and weighed anew the value of the specific and generic characters, and the 

 result is a renewed conviction that D. Smithii is a very distinct species and that 

 Diplothyra possesses in its umbonal plates a good generic character. 



Note on Penicillins. 



In a review of my work on Pholadacea, published in the American Journal 

 of Science, Prof. Gill suggests that Penicillinx be elevated into a family, 

 distinct from Gastrochscnidif, on account of the development of tentacles on 

 the mantle-margin of the animal of the former. I have reviewed the subject 

 and find no reason to alter the conclusions to which I originally arrived, 

 namely, to regard this difference in the animal, in connection with differences 

 in the shell, as possessing the value of a subfamily. Indeed, Prof. Gill has 

 himself separated, as a subfamily only, (Ceriplia&insf) our American JfeZonum*, 

 which differ from the exotic species in having a mantle with a plain instead of 

 fringed or tentaculate margin. 



Descriptions of two new Species of Fresh Water Mollusca, from Panama. 

 BY GEO. W. TRYON, JR. 



1. PLANORBIS FIELDII, Tryon, plate 1, fig. 4, 5. 



Description. Shell small, moderately thick, polished, unmarked excepi by 

 growth lines. Whorls three, almost equally convex above and below, and 

 rapidly enlarging. Spire not much depressed, umbilical region of moderate 

 width and not deep. Outer lip regularly rounded and almost equally expanded 

 above and below the plane of the volutions. The aperture is slightly oblique. 



Dimensions. Diam. maj. G, min. 4 millim. Alt. 2J millim. 



Hob. Panama. Capt. Field, U. S". N. Coll. A. N. S. ; my coll. 



Observations. This species resembles some varieties of PL deflectus, Say, in 

 which the whorl is not deflected at the aperture ; but it differs in the sides being 

 regularly rounded instead of carinate, as in that species, and also in the dila- 

 tion of the aperture above and below the plane of the shell, in this respect re- 

 sembling somewhat PL corpuhntus, Say. 



PL Panamensis, Dunker, is a very different shell from Fieldii, being more de- 

 pressed, differently sculptured, etc. 



Capt. Field presented to the Academy about a dozen specimens of this 

 species, together with a new Amnicola (herein described,) and a few specimens 

 of Succinea recisa, Morelet, all collected by himself at Panama. 



2. AMNICOLA PANAMENSIS, Tryon. plate 1, f. 6 



Description. Shell conical, smooth and shining, consisting of four rapidly 

 increasing, very convex whorls. Sutures deeply impressed ; spire prominent, 

 apex acute. Aperture rounded ; umbilical region slightly perforate. 



Dimensions. Length 4j millim. Diameter 3 millim. 



Hab. Panama. Capt. Field, U. S. N. ; coll. A. N. S. 



Observations. This shell is very like A. decisa, Hald., in form, but the whorls 

 are more convex and the aperture nearly rotund. It also resembles A. Cincin- 

 natiensis, Anth., which is, however, a more slender species. The shell is smaller 

 than either of the above. 



T think this is the first species of Amnicola found upon the Isthmus. 



