206 Royal Society : — 



of common salt nearly of the specific gravity of sea-water, viz. 1026; 

 and also by a weaker solution of specific gravity 1016. 



Finally, in reference to the inquiry regarding the distribution of 

 the species of fishes, he expresses his belief that some of the results 

 may be of useful application, especially those given in the second 

 and third sections ; inferring, that as in moist air, the vitality of the 

 ova is capable of being long sustained, they may during rain or fog 

 be conveyed from one river or lake to another adhering to some part 

 of an animal, such as a Heron or Otter, and also during a time of 

 snow or frost ; and, further, that other of the results may be useful 

 towards determining the fittest age of ova for transport for the pur- 

 pose of stocking rivers, and likewise as a help to explain the habitats, 

 and some of the habits of the migratory species. 



" Brief sketch of the Anatomy of a new genus of pelagic Gaste- 

 ropoda, named Jasonilla." By John Denis Macdonald, Esq., R.N. 



This communication refers to a remarkable genus of pelagic Gas- 

 teropoda, characterized, like Macgillivraya and Cheletropis, by the 

 presence of ciliated cephalic appendages, but having, as in the pre- 

 sent instance, a beautifully transparent, cartilaginous and perfectly 

 symmetrical shell. The author has seen but one species, which was 

 frequently taken between Port Jackson and the Isle of Pines. 



The shell resembles that of Argonavta in shape, is less than one- 

 eighth of an inch in diameter, and the little animal, when fully re- 

 tracted, occupies but a small portion of its cavity. The margin of 

 the mantle is of considerable thickness, containing loosely-packed 

 cells, similar to those of the middle or operculigerous lobe of many 

 Pteropods. About eight ciliated arms, identical in character with those 

 oi Macgillivraya, &c., encircle the head, including the mouth, which 

 is furnished with two massive lateral jaws bearing sharp prominent 

 dental processes on the anterior border, and with a pair of simple 

 tentacula having a dark ocellus at the outer side of the base of each. 

 A well-formed foot arises by a narrow pedicle from the under surface 

 of the body, immediately behind the ciliated collar. The creeping 

 disc is elongated in form, subquadrate in front, and tapers off gradu- 

 ally towards the posterior extremity. The latter part, correspond- 

 ing to the operculigerous lobe of other species, is speckled with 

 little clusters of dark pigment-cells, disposed so much after the 

 manner of those of the ciliated arms as to lead to the impression that 

 it is one of the same series, or whorl of organs, to use botanical 

 phraseology. A pectinate gill extends beneath the mantle, along 

 the anterior third of the dorsal region, lying, as in most cases, in 

 advance of the heart. The visceral mass of the body, though elon- 

 gated, is but slightly curved upon itself, not exceeding half a turn. 

 The lobules of the liver, distended with large amber-coloured oil- 

 globules, may be distinctly seen through the transparent outer enve- 

 lope and shell. Single spherical otolithes are contained in the 

 acoustic sacs, and the lingual ribbon is lengthy and flexuous, pre- 

 senting a row of uncini on each side, with a series of minute 

 denticulations, pointing backwards on their anterior and posterior 



