APPENDIX. 131 



Fig. 4 shews the situation of the young, before their extru- 

 sion from the parent shell. They are completely formed, and 

 covered with a shell before their ejection. 



Genus V— PISIDIUM. 



PiSIDIUM OBLIQUUM, pi. XXVII, f. 1, 2. 



Animal similar to that of Cyclas, but with the mantle ex- 

 tended posteriorly into a short, simple, contractile syphon; foot 

 tongue-shaped, and capable of much extension, a the syphon, 

 b the foot. 



Fig. 2 exhibits the situation occupied by the young before 

 their extrusion from the parent shell ; as they are viviparous as 

 well as the young of Cyclas. 



Genus CYCLAS.— (Page 116.) 



5. Cyclas citrina, pi. XXIV, f. 15, 15. 



Cyclas Jlavescens ? Macgillivray, Moll. Ab., p. 246. 



Shell very thin, subdiaphanous, and slightly elliptical ; very 

 ventricose, a little inequilateral, nearly hemispherical; umbones 

 large, prominent, inflated, and rounded ; covered with a rather 

 dull citron-coloured epidermis, beneath which the surface is 

 irregularly and strongly striated concentrically, with two or 

 three hues of growth. Length about two and a half eighths of 

 an inch. 



This shell dilFers from Cyclas cornea, in being more orbicu- 

 lar, in the umbones being much larger, more prominent, and 

 bulging, and it never attains so large a size as that species. 



Discovered by Thomas Glover, Esq., of Smedley Hill, Man- 

 chester, in the Leven, a little way below the Lake of Winder- 

 mere, Westmorland. 



