Mollusca of Great Britain. 381 



weeks, the young, with their testaceous coats completely 

 formed, and about the size of a pin's head, burst their en- 

 velope, and immediately enter upon their new functions, 

 swimming and walking with great activity. 



2. Hypnorum. 



Animal nigrescens. Tentacula supernfe albida. Susten- 

 taculum latum. 



Testa elongato-fusiformis, subventricosa, acuminata, fragilis, 

 diaphana, polita, fulvo-cornea. Anfractus 7, valdh tur- 

 riti. Apertura oblonga. 



Long. 0.5. — Diam. 0.2, 



Physa hypnorum. Drop. Hist, des Moll. p. 55. t. 3. 



/. 12, 13. 



Bulla hypnorum. Linn. Si/st. Nat. 1. p. 1185. 



Planorbis turritus. Miill. Verm. 2. p. I69. 



In slowly-running streams in many parts of Great Bri- 

 tain. Some specimens before me, from the garden ponds 

 of Fremington-house, Devonshire, are of unusual size, 

 measuring full three quarters of an inch in length. I have 

 only found them in the hottest days of the summer and 

 autumn months on the surface of the water, where they lie 

 floating with their shells downwards. 



Genus XV. PLANORBIS. Miill. 



Animal tentaculis filiformibus, acuminatis. Sustentaculum an- 



tich integrum, rotundatum. 

 Testa depressa : anfractibus cylindraceis : spirA vix unquam ex- 



sert4 : aperturd subrhomboide^, marginibus inaequali- 



bus : peristomio simplici. 



* Anfractibus 



