160 Dr Grant on. tJie Ova of the Poniobdella muricata. 



agree. But it is obvious enough that his results are not quite 

 accurate. Were we to correct them by substituting the true 

 atomic weight of nickel they would give us. 



Nickel. Oxygen. 



Protoxide composed of - . 3.25 -f. 1 

 Peroxide of - 3.25 + 1.7 



numbers approaching pretty nearly to those given by RothofF. 

 Upon the whole, I see no reason to doubt that we are in 

 possession of the true atomic weights both of nickel and co- 

 balt. 



Art. XXXIV. — Notice regarding the Ova of the Poniobdella 

 muricata. Lam. By R. E. Grant, M. D., F. R. S. Edin., 

 &c. Communicated by the Author. 



The ova of the Poniobdella muricata or Skate-leech, (Fig. 6 

 and 7. PI. II.) consist of small dark coloured spheres attached 

 by a narrow twisted neck to a spreading membranous base. 

 Each sphere consists of a double capsule containing a thick 

 gelatinous fluid, in the middle of which the young animal lies 

 coiled up generally in one spiral turn. The ova have much 

 the appearance of the seeds of the black pepper, being about 

 two lines in diameter, and having a rough and dull external 

 surface. On the opposite sides of the spheres there are two 

 round prominences, which fall off when the animal is come to 

 maturity, and leave two circular apertures to admit the sea 

 water, or to allow the young Poniobdella to escape. The outer 

 capsule, which has a dark greenish brown colour, is thicker 

 and more porous than the inner membrane, which is smooth, 

 thin, and very tough. The stalk which connects the sphere 

 to the flat base is solid, of a black colour, and firm consistence 

 internally, and is strongly marked with longitudinal promi- 

 nent ridges, which have a twisted and fibrous appearance. The 

 spreading base which adheres to stones, shells, or other hard 

 bodies, consists of the same tough dark-coloured substance 

 as the outer layer of the ovum and the stalk, and is marked 

 with elevated radiating ridges like diverging roots. The ge- 



