154 NEAV UNIONID^, MELANIDiE, ETC 



, j..j.jjijxi.j., -..iv^ij, JJJ.V/., 



Remarks. — This is a very remarkable shell, and I have placed it among the Pla- 

 norhes until the soft parts may be observed in a living state ; they may be found to 

 differ from the true Planorbes* Some specimens preserved in alcohol have been 

 carefully examined, but the parts are so rigid that it could not be satisfactorily done. 

 The tentacula do not, however, seem to be so long as is usual in the Plaivyrhes. The 

 epidermis is very thin on the upper part of the whorls, and the strise there are back- 

 wards in curves, and on the lower part slightly forwards. The upper carina forms 

 an acute angle, the edge being cord-like ; the lower one is still more acute. In most 

 of the specimens there are two obscure carinations on the whorls between the acute 

 ones. The umbilical region is very remarkable, the perforation extending to the 

 apex of the slightly elevated spire, the apex itself being frequently wanting, owing to 

 corrosion occasioned by the attacks of some small enemy eating into the substance of 

 the hard part. The upper angle of the whorls is elevated slightly above the plane of 

 the whorls, thus forming a babjdonic appearance. This gives the shell the appear- 

 ance of some forms of the Trochi. This very curious and interesting species is among 

 the Mollusca brought by J. S. Newberry, M. D., attached to the Pacific Rail Road 

 Survey under the command of Lieut. R. S. Williamson, U. S. A. ; and I have great 

 pleasure in dedicating it to Dr. Newberry, who has done so much to elucidate the 

 Natural History of California and Oregon, when on these expeditions so creditable to 

 the Government. 



I received in February, 1856, from Dr. John B. Trask, of San Francisco, California, 

 a number of interesting Molluscs, some of which are new to me. I propose to give the 

 diao'noses of these, and merely to enumerate the others which are already known, 

 adding what I believe to be their synonymy. 



Among the Unimiidce I found no new ones, and it may be mentioned that the 

 wenera and species of this family are remarkable, first, for their very small number, 

 secondly, for the absence of strongly pronounced characteristics ; there being no tu- 

 berculate, plicate, spinose or sulcate species yet observed, although many naturalists 

 within the last fifteen or twenty years have well examined the lakes and rivers of 

 the country. 



We are not the less surprised at the remarkable paucity as regards also the families 

 Colimacea, Lymnceana, Melaniana and Peristomata. This is most strikingly in con- 

 trast with the profusion which exists in regard to most of these Families, in the 

 waters which drain the eastern flanks of the Rocky Mountains, and the vast valleys 

 and plains intervening between them and the Atlantic Ocean. 



* Provisionally it may be called Megasystropha, from m«)-», magnus, and o-uirTg»<f)!, vortex, the umbilicus being 

 large and vortex-like. 



