24 SPH.«RIID^. 



scarcely any nacreous lustre ; other internal characters the 

 same as in P. fontinale. L. 0-175. B. 0-2. 



Var. ohtusalis. Shell smaller and much more ventricose ; 

 healcs prominent, very obtuse. P. ohtusale, C.Pfeiffer, Deutsch. 

 Land- und Sussw.-MoU. i. p. 125, t. 5. f. 21, 22 ; F. & H. ii. 

 p. 120, pi. xxxvi. f. ] . 



Habitat : Mossy swamps, shallow ditches, drains, 

 grassy pools, and similar situations throughout aU the 

 country ; and it is one of our upper tertiary fossils. It 

 ranges from Lapland to Corsica, and is generally diffused 

 in Europe. Dr. Baudon says that it is the same species 

 as the P. vejitricosum of Prime ; so that it appears to be 

 also a native of the United States bf America. The 

 variety obtusalis occurs in similar situations with the 

 typical form ; but it is more local and not so abundant. 

 The intermediate gradation between the two forms is 

 almost infinite ; but the essential and common character 

 of both is the same, viz. the beaks being nearly central 

 and obtuse. 



Having carefully studied the description and figure 

 given by Poli of his Cardium Casertamim, I have not 

 been able to arrive at the same conclusion which Moquin- 

 Tandon and other French conchologists have formed, 

 that this species is the type of the one which I have de- 

 scribed as P. fontinale ; and I consider that it ought 

 more properly to be referred to the present species. Poli 

 says his shell is " subrotunda f' and his figure shows 

 that it is much more equilateral than P. fontinale. He 

 also remarked the irregularity of the strise arising from 

 the marks of groAvth, which appears to be more cha- 

 racteristic of this than of the other species. As, how- 

 ever, he did not notice any other species, it is of course 

 very difficult to say precisely which species he meant ; 

 and under these circumstances I think it is preferable 

 not to revive an obscure and doubtful species, indicated 



