\ 



TESTACEA. 



Planorbis imbricatus. — Plate XI. Figs. 7, 8, 9, 10. 



Diameter of the shell a line ; volutions three, with transverse spinouf? 

 ridges at intervals formed around the tortion of the epidermis. The 

 intervals between the ridges are hollow. When free of impurity, the 

 shell is grey or of a reddish tinge. Animal a Limax. Tentacula two, 

 almost cylindrical, obtuse, incurved during progression. A black eye 

 tenants the front of the head at the base of each. Sole oval. Colour of 

 the animal dark grey. It feeds on vegetable substances, including the 

 bark of beech and other trees lying in ponds and canals. 



This creature spawns in February and in May. The spawn consists 

 of very minute spots, generally containing a single embryo, and some- 

 times two. I have not at any time observed more. The spawn is im- 

 perceptible, unless when affixed to the side of a glass vessel. 



Specimens have survived above eighteen months in confinement. 



Plate XI. 



Fig. 7. Planorhis imbricatus. 



8. The same, enlarged. 



9. Spawn. 



10. The same, enlarged. 



Valvata obtusa.— Plate XI. Figs. 11, 12, 13, 14, 15, 16, 17, IS, 

 19, 20, 21, 22, 23, 24. 



Strictly speaking, this is not one of the logarithmic curved, flattened 

 shells ; it more resembles the Helixes of our fields and gardens. 



Diameter of the shell a quarter of an inch, height nearly as mucli. 

 Volutes four, rising gradually in conical arrangement above each other, 

 and concave below. Orifices circular, closed by an operculum. Animal 

 a Limax. Length above a quarter of an inch ; head cylindrical, distinct. 

 Sole, with a broadened .shoulder recurving to each side, like some of the 

 P]olidcs in the Doris tribes. Shortened extremitv obtuse. A branchial 



