1881.] 



NATURAL SCIENCES OF PHILADELPHIA. 



95 



It may here be remarked that Macgillivra}^, an enthusiastic 

 observer, Avho has described some of the. British species with 

 great fidelity, lays much stress on the shape of the mouth, com- 

 paring it with those of Helix and Zonites, as conclusive of the 

 dexti-al character of the shells in Planorhis} 



If analogies in form of mouth are worthy of consideration, 

 though this point may not have much weight without otlier and 

 corroborative evidence, we have in this character a stronger argu- 

 ment the other way, b}-, more propeiiy a comparison between 

 nearer related forms like Physa (that is the more globose species), 

 and most of the forms of Planorhia I have given, holding the latter 

 in a normal sinistral position, when the tendency to the ph^^soid 

 moutli, tlie ovate shape and sag of the aperture will be readily 

 noticed. 



Frequently, immature, half-grown, and less than half-grown 

 shells of Planorhis have been brought to me by collectors who 

 were quite confident they had made new discoveries, and it is not 

 improbable that j'oung shells as above have been described and 

 published as new species of Physa. 

 Fig. 6. 



Fig. 5. 



Fig. 7. 



Ameria scalaris, Jay. Physa globosa, Physa hiimerosa, Gld. 

 Hald. 



Fig. 8, 



Fig. 9. 



Physa ancillaria, Say. 



Physa ampullacca, Gld. 



I would further 

 suggest a compari- 

 son of the apertures 

 of our larger Cali- 

 fornian (adult) 

 shells of Planorhis^ 

 held in a sinistral 

 position with Ame- 

 ria scalaris (fig. 5)^ 

 (= Paludina sca- 

 laris, Jay), a curious 

 Florida form ; Phy- 

 sella globosa (fig. 6), 

 Hald., a Tennessee 



^ It is presumable that the shells of Planorhis, by which Macgillivray 

 was impressed and which were the most familiar to him, were the small 

 species of his own country, which are flat, symmetrically coiled, regular in 

 form, and gradual in growth, being in striking contrast with the sturdier, 

 ventricose West American forms I have cited— which also more conspic- 

 uously exhibit sinistral characters. 



- Dall says : "A careful examination of a number of specimens of this 

 singular form, shows that it is distinct, and not a young Planorhis, as has 



