98 



PROCEEDINGS OF THE ACADEMY OF 



[1881. 



Fig. 11. 



Plan, tumidus—nnoleixr whorls. 



P. Traskii, Lea, whicli Mr. Binney makes a synonym of P. 

 ammon, belongs to the western corpulentus form. 



While the forcooing figures (10) show the shells of one species 

 reduced to one and a-half whorls, the following figures exhibit the 

 form of Planorhis tumidus, Pfr., which measured in 



Largest diameter, . . . . '68 inch, 

 with five whorls, reduced by breaking down to two and a-half 

 whorls and a diameter of '16 inch. 



This last is a more southern 

 species ;. numerous specimens were 

 collected by the late Thos. Bridges, 

 who found them abundant in ele- 

 vated pools, small lakes, etc., amid 

 the forest slopes of Mombacho, in 

 Nicaragua. It closely resembles 

 more northern forms and should 

 hardly be called a species. 



It will be observed in the figures 

 (10) that we have in the embryonic shells of Planorhis corpulentus 

 a near approach to Physa, a, close resemblance to a Physa with a 

 flattened spire ; suggesting such physoid forms as P. humerosa, 

 while the adolescent stage of Planorhis tumidus (at half its adult 

 size) also suggests an umbilicated Physa with a flattened spire, 

 somewhat like (N. W. American) Physa Lordi (fig. 12), with the 



spire cut off, and an umbilicus 

 punched in, back of the mouth. 



The first figures (10) explain 

 De Kay's " Physa jAanorbula " 

 (fig. 13), and also suggestively 

 point towards Ameria scalaris. 

 These figures also exhibit the phj'soid mouth, and 

 show that there is neither necessity nor propriety 

 ysa or i. .^ leaving closely related forms for more distant, 

 analogies. In this connection it should also be kept in mind that 

 certain species of Physa, included in Ehrenberg's subgenus 

 Isido7'a, are more or less umbilicated. Of the smaller species of 

 Planorhis which have passed under ray examination, I have seldom 

 found it difficult to determine the sinistral characters b}^ a com- 

 parison of the two sides of the shell. 



If we could unroll a specimen of, say, Planorhis sjnrorhis, and 



Fig. 13. 



Physa 2)lanorbula. 



