272 Proceedings of the Royal Society. 
Thursday, May 6. 
Lieutenant Henry Forster, R.N., was elected a Fellow of the 
Society, and being about to leave England on the Polar Expedi- 
tion under Captain Parry, he was immediately admitted by the 
President. 
The following papers were read :— 
On Univalves. By Charles Collyer, Esq. Communicated by Sir 
James Macgrigor, F.R.S. 
In this paper the author entered into a series of details respecting 
the structure of shells, and the anatomy of their inhabitants, 
which he thinks suggest the necessity of separating the natural 
history of the former from that of the latter. By parts peculiar to 
univalves, he proposed to distinguish and nominate families, to 
divide into sub-genera such as are distinguished by an uniform 
state of the more general feature, and to separate into individuals, 
such as with this particular state have additional parts, or modi- 
fications of such parts. To render the nomenclature perspicuous, he 
suggested Latin derivations of one termination, expressing some es- 
sential distinctive feature, or difference of colour or size; and where 
these fail, he had recourse to similitudes with other objects. The 
parts or conditions chosen for generic distinction and denomina- 
tion, are cavity, lip, columella, rostrum, and spire, open, tubular. 
The application of these principles of distinctive description was 
illustrated by reference to several individuals, such as Argonauta, 
Cyprea, Conus, Trochus, and others. Shells that are partly or 
completely open and flat present, said the author, no feature for 
association, and:-hence a condition must be chosen, namely, the 
presence or absence of a margin. Lastly, the author divided tu- 
bular shells into straight and open, straight and closed, and con- 
torted. 
Of the effects of the density of Air on the Rates of Chronometers. 
By George Harvey, Esq. Communicated by D. Gilbert, Esq., 
F.RS. 
Among the sources of error to which chronometers are liable, 
