On the Embryogeny of Lamellaria perspicua. 119 



have small pearls of this texture adhering to the centre of 

 each valve. 



As a guide to the form of this species, I may mention that 

 the figure of Yoldia Woodwardi in the ' Thesaurus Conchy- 

 liorum,' vol. iii. pi. 226. f. 22, gives a very fair idea of it, 

 except that the umbo is situated too near the acuminate end. 



XV. — On the Embryogeny of Lamellaria perspicua. 

 By M. A. Giard*. 



The recent researches on the embryogeny of the Pectini- 

 branchiate Gasteropods relate to a veiy small number of types — 

 Paludina vivipara (Leydig), Calyp>trcea sinensis (Stefanoff 

 and Salensky), and Purpura lapillus (Selenka). It was not, 

 therefore, useless to undertake the study of the development 

 of a sufficiently abnormal group, that of the Sigaretida?. 



Lamellaria perspicua lays its eggs at Wimereux during the 

 months of February and March. This mollusk hollows out 

 its nest in the colonies of the compound Ascidia, from which it 

 derives its nourishment (Leptoclinum maculosum and Poly- 

 clinum succineum). The nest has been seen and described by 

 Kennedy and Peach. I will only add that the transparent 

 operculum, which closes it, presents circular and concentric 

 strise, indicating that the female turns on herself during ovi- 

 position, as also do a large number of nudibranchiate mollusks. 

 Each capsule contains, besides the normal eggs, a certain 

 number of rudimentary eggs, which serve at a later period for 

 the nourishment of the embryo. The ovarian egg presents a 

 vitelline membrane ; the deposited zgg is quite destitute of it. 

 Its contents are formed chiefly of fatty globules, which do not 

 allow one to see the germinal vesicle. Just as segmentation 

 is about to commence, a spot of a dull white colour appears at 

 the surface of the egg, to disappear soon after. The egress of 

 the polar corpuscles could not be observed. 



The egg separates into two parts, of which the largest divides 

 in its turn into two and then into three. We have thus four 

 spheres — namely, a large one (the still undivided primitive 

 sphere) and three small ones. These four spheres are not 

 arranged in a cross, but in a tetrahedron, like four cannon- 

 balls forming a pile. In the portion situated between the 

 points of contact of the four spheres, each of them gives birth 



* Translated by \V. S. Dallas, F.L.S., from the 'Comptes Rendus,' 

 22nd March, 1875, p. 73G. 



