104 Rev. L. Jenyns on the smaller British species q/" Pisidium . 



Fig. 39. Egg of Nais albida {ad nat. del.), showing — a, shell; b, chalaza; 

 c, yelk-bag ; d, d, two yelks, each of which is now triglobular. 



Fig. 40. Ditto, (htto, with one division only fissurating, and small cells 

 without apparent lissuration appearing in the other two, which 

 have become united. 



Fig. 41. Ditto, ditto, ditto, more advanced. 



Fig. 42. Ditto, ditto, ditto, with the two unfissurating lobes undistin- 

 guishably united, and forming one-half of the embryo, while the 

 other lobe forms the other half, and is still further advanced in 

 fissuration. 



Fig. 43. Ditto, ditto, ditto, uith fissuration complete, and the halves still 

 united, but foreshadowing the line of separation, which extends 

 from the notch inwards. 



Fig. 44. Ditto, ditto, with the two embryos fully developed. 



Fig. 45. Ditto, ditto, showing the development of sacs in the yelk, and 

 subsequent production of monads ; a, a, sac filled with granular 

 matter passing into monads ; h, b, sacs with fully developed mo- 

 nads leaving them ; c, c, c, sacs from which all the monads have 

 escaped ; d, tubular form, in which the sac appears outside the 

 shell of the ovum ; e, e, effete contents of the yelk. 



Fig. 46. Monads, more magnified (less than l-5600th of an inch in dia- 

 meter) : a. just after exit ; b, after loss of the cilium and com- 

 mencement of tubulation. 



Fig. 50. Filnria frequently fouiul singly, or in more or less plurahty, in the 

 peritoneal cavity of Nais albida : a, oesophagus in a sheath ; 

 b, intestine enclosed in the hepatic sheath j c, anus; d, vulva; 

 e, p, double ovisac. 



X. — Note on the smaller British species of Pisidium. 

 By the llev. L. Jenyns, M.A., F.L.S. &c. 



Since the publication of my monograph on the British species 

 of Ci/clas and Pisidium in 1832*, I have received at different 

 times, through the kindness of correspondents, many specimens 

 of the smaller species of the latter of these genera, from various 

 localities, for identification and comparison. Having lately had 

 occasion to re-examine some of these more closely, I am induced 

 to offer a few remarks respecting them for the benefit of those 

 naturalists who interest themselves with these little raollusks. 



Pisidium Henslowianum. — Fine specimens of a shell, measuring 

 nearly 22- lines in length, and exactly agreeing with this species, 

 except in being entirely destitute of the lamelliform projection 

 on the umbones, have been sent me from Ackworth, near Pon- 

 tefract, and others from Ambleside, by Mr. J. W. Watson of the 

 former place. From the absence of the appendage alluded to, 

 I was for some time doubtful whether I was right in referring 

 these shells to the P. Henslowianum, or whether they ought not 

 more properly to be considered as large individuals of P. pul- 

 chellum. But on lately giving them a closer inspection, I per- 



* Camb. Phil. Trans, vol. iv. p. 289. 



