300 Mr Jbnyns's Monograph on the 



authors. Draparnaud appears to be the first who made a distinction 

 between this shell and the C. calyculata, and this distinction has been 

 since acknowledged not only by Lamarck, but also by Pfeiffer and 

 Nilsson ; but all the British specimens that have fallen under my ob- 

 servation with the name of C. lacustris attached to them, are in my 

 opinion nothing more than mere varieties of the species under con- 

 sideration. 



As Muller has described only one of these two species, I consider 

 it doubtful to which his description applies. I have therefore made 

 no reference to his work in the present instance. 



C. calyculata is much less abundant in this country than C. cornea. 

 Montagu met with it in Devonshire and Wiltshire. Mr Alder finds 

 it near Newcastle, but says that it is rare ; and it has occurred sparingly 

 to myself in two or three parts of Cambridgeshire. — Var. /3, I observed 

 last summer (1831) in considerable abundance in one pond on Bookham- 

 Common in Surrey, and some which I kept by me alive for a few days, 

 showed more activity than the last species, readily and frequently as- 

 cending the sides of the vessel, and walking, like Physa Hypnorum, 

 on the under side of the surface of the water*. Occasionally they re- 

 mained in a quiescent state at the bottom with their posterior extremity 

 elevated, and the siphonal tubes exserted to a considerable length, often 

 nearly equalling that of the shell itself. — Var. 7. I have received from 

 the North of England. 



In young specimens the tubercle at the apex of each valve, so charac- 

 teristic of this species, is relatively much larger than in the adult shell. 



* This phraseology is not strictly correct, but is perhaps sufficiently intelligible. The 

 action intended, consists in the animal extending its foot along the surface of the water 

 with its shell immersed, and in an inverted position. In this manner, it contrives to 

 traverse the vessel from side to side as though it were crawling along a solid plane. 



