1855.] CJiusan Shells. 125 



Nepa, and covered with Lemna and Chara, yet it is mixed with salt- 

 water in the vicinity of the sea, which may account for the appear- 

 ance of marine Crustacea and Testacea." 



"Whorls 4 in number. This shell seems to take its stand between 

 the depressed Planorbes (as marginatus, earinatus, spirorhis, vortex 

 etc.) in which the angle of the penultimate volution scarcely enters 

 the mouth of the shell, and the Sylhet species umbilicalis* the 

 English nitidus, the Bengal trodwides, in which the angle of the 

 penultimate whorl projects far into the cavity of the mouth. In its 

 compressed form it assorts with earinatus, while in the character just 

 noted, in the great comparative breadth of the last whorl, and in the 

 somewhat contracted umbilicus, it approaches to umbilicalis. The 

 arcuated and nearly semicircular upper lip forms a bow, of which the 

 straight lower lip replaces the chord, and joining on to it at the 

 sharp periphery, gives the mouth a very singular appearance. 



In this description, I have considered the shell as dextral. On a 

 former occasion I gave my reasons with reference to the position of 

 the animal in the shell, and Mr. Gray, who quotes my observations, 

 states that Mr. Desmoulins, who has examined the question in detail, 

 concludes that the shell of Planorbis is essentially dextral, and that 

 a displacement to the left side of the extremities of certain organs 

 which are themselves on the right side, has led to the erroneous 

 opinion, derived from imperfect anatomical investigation, that the 

 animals were sinistral. Swainson, somewhat strangely seemed to be 

 unaware of this investigation, when he noted that one of the cha- 

 racters of Planorbis was a reversed aperture. I have not referred this 

 shell, nor the next, to any of his sub -genera of Planorbis. He evi- 

 dently has not worked them out, nor traced their analogies to the 

 families of the Phytophaga. His sub-genera Planorbis and Heli- 

 soma appear to be scarcely distinguishable by their descriptions, as 

 will appear from the following table, in which I have merely trans- 

 posed all that Swainson says regarding them, for the sake of the 

 juxta-position of the characters, putting my own observations within 

 brackets. 



* Dr. Cantor has lately discovered at Serampore and Barrackpore a new spe- 

 cies, belonging to this type of form which is closely allied to umbilicalis. 



