Davidson—Recent and Tertiary Species of Thecidium. 15 
constitute M. E. E. Deslongchamps’ ‘ascending apparatus,’ the cen- 
tral portion (F and G) being likewise more elevated than the other 
parts, and overlie a portion of the visceral cavity. 
The parallel grooves, or spaces left between the ridges above de- 
scribed, are partially occupied by a lamella, in the shape of a double 
crescent (¢ and), of which the larger branches (7) partly occupy the 
large cavities left between the first and second ridges; these being on 
their inner sides intimately united with the sides of the grooves ; 
while the shorter branches (f) are freely suspended over the visceral 
cavity and occupy the spaces left between the third and fourth 
ridges of the ascending apparatus. To these crescent-shaped lamella 
M. E. E. Deslongchamps has given the name of ‘descending ap- 
paratus.’ 
The interior of the ventral valve is concave and deep, with asmall, 
longitudinal, rounded, mesial elevation ; the hinge-line is straight, 
and on either side at the base of the deltidium are situated strong 
hinge-teeth for the articulation of the valves. ‘The beak is hollow, 
but there exists on the median line, and far back in the cavity, a 
small elevated septum, to each of the sides of which is attached a 
very thin small concave plate (Pl. I. fig. 3-6), to which, according 
to M. Duthiers, the adductor muscle, or ‘occlusor’ of Hancock, was 
attached. On the bottom of the valve, a little lower down, and on 
either side of the mesial elevation, and partly under the cavity of 
the beak, a large pyriform scar is observable, to which the ‘ divari- 
cator muscles’ of Hancock were attached (c) ; and, lastly, outside of 
these on the bottom of the valve, and near the angles of the hinge- 
line, there exists a small oval scar (a), which is believed to have been 
produced by the ‘ ventral adjustor’ of Hancock. ‘The whole remain- 
ing surface of the interior of the shell is closely covered with numerous 
granulations or asperities. 
Such are the principal shell-characters of the recent Thecidium 
Mediterraneum ; and, although the comparisons we shall shortly en- 
deavour to establish between these and the fossil species cannot 
extend to the animal itself, it will be as well to refer to some few 
observations upon the subject, especially in relation to the muscles, 
mantle, and oral arms, which we will in a great measure extract 
from M. Lacaze Duthiers’ interesting memoir, as these details cannot 
fail to prove of interest to Palzontologists, ‘and have not yet been 
alluded to in any English work. 
Relative to the muscles, M. Duthiers informs us that there are three 
pairs, six muscles in all, possessing distinct directions and functions. 
Of these six muscles, two (Pl. II. figs. 7, 8, 9, a@) are lateral, short, 
wide, and easily observable, and serve without doubt for the oc- 
clusion of the shell. These are M. Duthiers’ ‘muscles adducteurs 
lateraux ou externes,’ and which, Mr. Hancock informs me, would 
represent his ‘ adjustors,’ but that the function of this pair of muscles 
does not appear to be exactly similar to the ‘adjustors’ of Wald- 
heimia, in which they move the shell upon the pedicle; for in T’he- 
cidium they assist at once in closing the valves, and in preventing 
