344 Ichthyology of the Firth of Forth. 
off to a place of safety, commonly beneath the shelter of a neighbour- 
ing crevice, of such limited dimensions at its entrance, that it seems 
surprizing it should ever be able to return from it, in its new state 
of growth. The first instinct of nature after securing a place of 
retreat, is to absorb, I suppose by swallowing it, as much fluid as 
will distend its organs and their common covering, now as flexible 
as velvet, to the full extent of their capacity ; by which means the de- 
position of crustaceous particles is made according to the dimensions 
of its newly acquired bulk : which in the instance now described, 
that had arrived at the ordinary size of a female, and allowing for 
the thickness of the first crust, exceeded the former by at least an 
inch in its longest diameter, and in all its dimensions in the same 
proportion. In the early stages of growth, the increase of bulk at 
exuviation is, of course, much more considerable, in relative pro- 
portion. Some of these smaller specimens may be found passing 
through this natural process in every month of the year ; but in 
some adult individuals, the same crust must continue for a long time, 
since I have seen one with oysters measuring two inches and a half 
in length, on the carapace ; and in another, a muscle (mytilus) about 
an inch in length, attached by its byssus to a joint near the body. 
Whether, as has been said, the rejected case forms a meal for its 
old inhabitant, on its first return to habits of activity, may well be 
questioned ; though I have known an instance where one crab has 
devoured another, leaving nothing but the points of the legs, and a 
small portion of the carapace ; but in the analogous instance of a 
prawn, that had just thrown off its exuviae, I have detected the 
source of the firmness of the new covering, in the presence of small 
shell-fish in its stomach. Under a lens, a minute trochus, perhaps 
the T. zizyphinus, and some specimens of an oval bivalve, were dis- 
tinguishable. This is perhaps the only period of their lives when 
their food is of this kind, but its nature cannot often be ascertain- 
ed, by the comminution it undergoes from the manner in which it is 
devoured, when the jaws have attained their firmness. 
Polperro, 1836. 
IV. Contributions to the Ichthyology of the Firth of Forth. By 
RICHARD PARNELL, M. D. 
No. II. The Tadpole Fish. 
THE object of this paper is to shew, that the Raniceps trifurca- 
tus is by no means so rare a fish as naturalists in this country have 
supposed, and to point out some very important characters in it, 
which have escaped their attention. 
