AND ALLIED FORMS. 29 



II. ON THE SUBFAMILY POMATIOPSINil. 



Genus unicum POMATIOPSIS, (Tryon) Stm. 



In the Proceedings of the Philadelphia Academy of Natural 

 Sciences for September, 1862, page 452, Mr. Tryon proposed to 

 separate from Amnicola a group of elongated species, as a sub- 

 genus under the name of Pomaiiopsis, with the following diag- 

 nosis : — " Shell elongate, the spire (of about six whorls) much 

 exceeding the length of the aperture. Example, A. lapidaria, 

 Say." Following the diagnosis above this name could not be 

 adopted for the terrestrial genus now to be described, for there 

 are elongated species and ovate species in both the terrestrial 

 and aquatic groups of the old genus Amnicola. But as Mr. Tryon, 

 in accordance with a correct practice which authors would do 

 well to follow universally, has distinctly mentioned "A. lapida- 

 ria^^ as the type of the genus, I do not hesitate to adopt his name. 

 Prof. Gill, in his paper already alluded to, doubts the validity 

 of the subgenus as defined by Mr. Tryon, although he recognizes 

 that the type " may however be quite different, and a representa- 

 tive of the Aciculidse." But the Pomatiop>sis lapidaria, as I shall 

 presently show, is, notwithstanding its terrestrial habits, by no 

 means allied to the terrestrial Pneumonopoma to which the Acicu- 

 lidee belong according to the observations of Moquin-Tandon. 

 The mollusks of that group have a vascular respiratory cavity or 

 lung, and their tentacles have the power of erection and motion 

 during land-progression; while our Pomaiiopsis breathes by 

 means of a pectinated gill, and has no power of raising its ten- 

 tacles in air, though in water they are of course mobile. The 

 structure of its respiratory organ also separates this genus from 

 the Truncatellidte, which have nearly the same mode of pro- 

 gression. One genus, however, which has been referred to the 

 latter family, approaches Pomaiiopsis very nearly. I refer to the 

 Tomichia of Benson, an East Indian form, the respiratory organs 

 of which have not yet been observed. 



Pomaiiopsis is one of the very few true Ctenobranchiates 

 which have yet been discovered to breathe air, habitually if not 

 solely. Dr. Lewis, in his paper in the Proceedings of the Boston 

 3 



