234 BRITISH MOLLUSKS. 



siplional tube, which is at the same time extended, and kept so, is 

 short, cylindrical, truncate, and undergoes little alteration, a current 

 is seen passing out of it, and minute dark particles frequently escape. 

 In this manner, the animal advances with considerable speed by 

 jerks. At other times it ascends to the surface, where it proceeds 

 in the same manner with the shell reversed, the umboes being 

 beneath." 



On the Continent P. pulchellum ranges with P. Casertanum, 

 through the central and south parts. 



7. Pisidium Henslowianum. Senslow's Pisidium. 



Shell ; rather obliquely subtriangularly orbicular, ventricose, mode- 

 rately shining, yellowish horny, finely concentri- 

 cally striated, inequilateral, anterior side slopingly 

 produced, posterior arcuately rounded ; umboes 

 furnished with a lamelliform projection. 

 Tellina Senslowiana, Sheppard (1823), Trans. Linn. Soc. 



vol. xiv. p. 149, 150. 

 Cyclas appendiculata, Turton (1831), Man. p. 15. f. 6. 

 Fisidium acutum, Pfeiffer (1831), Wiegm. ArcMv. vol. i. 



p. 230. 

 Fisidium Hensloioianum, Jenyns (1833), Trans. Phil. Soc. 



Camh. vol. iv. p. 308. pi. xxi. f. 6 to 9. 

 Fisidium pallidum and Jaudonianum, Grassies (1849), Deser. Pisid. Aquit. 



pi. i. f. 10. 

 Pisidium Recluzianum, Bourguignat (1852), Journ. Condi, p. 174. pi. viii. 



f. 8. 

 Pisidium Bonnafouxiana, Cessac (1854), Desc. Nouv. Pisid. p. 6. 

 Pisidium Dupuyanum, Norrnand (1854), Coup d'ceil Cycl. p. 5. 

 Hah. Central Europe. England and Ireland. (In ponds and ditches.) 



The character upon which P. Henslowianum was founded as a 

 species, is that of each umbo being surmounted by a kind of cup- 

 like projection. These, as Dr. Gray observes, are evidently formed 

 on the edge of very young specimens, and gradually rise to the 

 umbo as the shell increases in size by the addition of new shelly 

 matter to its edge. An abnormal secretion of this kind cannot 

 be regarded as a specific character, the species is maintained by 

 Moquin-Tandon on other general grounds, and several varieties of 

 it are characterized, without any such appendage. 



