In the ovo-viviparous sharks, which are the subjects of the present
communication, the ova being contained till hatched in the body of
the fish, have no hard covering, and are surrounded by sea-water,
which has admission into the oviduct, by an aperture for that purpose.
In the kanguroo, and others of the opossum tribe in New South Wales, there is a communication between the uterus and vagina, which answers the same purpose of aerating the foetal blood, which is necessary in these animals; because the young is not, as in others, connected with the uterus by a navel-string, but is detached as a sort of soft egg, and consequently cannot receive the influence of the ar- terial blood of the mother through the coats of any contiguous vessels.
The calculus, which is the subject of the present essay, is the only new species which the author has had an opportunity of observing, in addition to five which he described to the Society in the year 1797.
It appears to be comparatively very rare, as he has hitherto seen only two specimens of it ; one in the possession of Dr. Reeve of Norwich, and the other in a collection of calculi belonging to Guy‘s Hospital. They are in appearance more like the triple phosphate of magnesia than any other calculus, but are more compact and semi- transparent, with a slight tinge of yellow.
By destructive distillation they yield fietid carbonate of ammonia, with a heavy animal oil, and the residuum is a black spongy coal.
They are not dissolved by water, by alcohol, by acetic acid, by tartaric acid, but are dissolved by most other acids, by muriatic, ni- tric, sulphuric, phosphoric or oxalic acids.
They are dissolved also by most alkaline menstrua, as by solutions of pure potash or soda, pure ammonia, or by lime-water, and even by the fully saturated carbonates of potash or of soda, but not by car- bonate of ammonia.
It is remarked also, that the solution formed with nitric acid does not yield oxalic acid, as the uric acid does when similarly treated, and does not turn red in drying, but becomes brown, and ultimately black, when much heated.
Since this substance yields carbonate of ammonia by distillation, it evidently contains oxygen, but it does not appear acid, as it does not redden litmus, but has rather the properties of an oxide, inasmuch as it unites readily with either acid or alkaline substances; and the author is induced to give it the name of cystic oxide, because the only calculi hitherto observed have been taken from the bladder.
The author takes this opportunity of correcting an inaccuracy or two in his former communication on this subject; and he also adds
