16 Davidson—Recent and Tertiary Species of Thecidium. 
any irregular or lateral movements, which, from the central posi- 
tion of the occlusors, might be liable to take place. In fact, he 
believes that the function of the ‘adjustors’ in Thecidium is modified 
much as it is in the same muscles of Lingula; and he is therefore 
inclined to conjecture that the two valves of the former are not so 
firmly hinged as they are in the other articulated forms. ‘The second 
pair (6 of the figures already named) are those nearest to the median 
line ; these are termed ‘muscles adducteurs internes’ by M. Duthiers, 
and correspond with, or are equivalent to, Mr. Hancock’s ‘anterior 
and posterior occlusors.’ In the ventral valve these muscles would 
be fixed to the bottom of the valve, nearly under the extremity of 
the tongue-shaped ‘ascending shelly process’ already described, and 
be again attached to the two thin shelly processes (Pl. I. fig. 3, 6) 
which we have described as existing in the cavity of the beak. The 
function of these muscles (as stated by M. Duthiers) would also be 
to effect a closing of the valves, but less efficaciously than the pre- 
ceding pair. The third pair, which M. Duthiers designates ‘mus- 
cles abducteurs’ (PI. II. figs. 7, 8, 9, ¢), are Mr. Hancock’s ‘ divarica- 
tors, and their function would be to open the valves. They form 
large impressions on the bottom of the ventral valve, on either side 
of the central septum (PI. I. fig. 3, c), and have their other end at- 
tached to the extremity of the cardinal process of the dorsal valve. 
M. Duthiers has had the opportunity of studying a vast number of 
individuals of this Thecidium in the living state ; and he mentions 
that, contrary to what we find to be the case in Terebratula, the 
animal opens its shell very widely; the dorsal valve rising on its 
hinge at right angles to the ventral valve, as the lid of a snuff- 
box (Pl. Il. figs. 9, 10). The animal is also sensible to light and 
darkness ; and it draws down its smaller valve with the rapidity 
of lightning at the approach of any danger. M. Duthiers observes 
that the mantle is exceedingly thin, and does not possess round 
its margin any of those long setz# which exist in Terebratula 
and Lingula, and that the genital lobe (the one which corresponds 
to the concave or ventral valve) is very different to the other, 
and possesses in its thickness, or in its central portion (that is to 
say, towards the deep and concave portion of the valve), very thick 
calcareous plates (Pl. II. fig. 6, 0), the analogues of the plates and 
spicula occurring in the mantle of TYerebratula; also that these 
plates in Thecidium participate in the character of the shell itself, 
being smooth on their under surface and covered on their upper side 
with asperities similar to those which cover the entire surface of the 
bottom of the valve. These plates are very thick, and form a ceiling 
or vault over the cavity which contains the organs of reproduction ; 
and they have been carefully described and figured by M. J. Bosquet, 
M. E. E. Deslongchamps, and myself, as seen in several fossil species, 
and in particular in Thecidium vermiculare from the Upper Creta- 
ceous beds of the Duchy of Limbourg. 
The so-termed ‘oral arms’ of Thecidium are stated by M. Duthiers 
to resemble those of other Brachiopoda, except that they are not 
free, but adhere in all their extent. He states that the swollen basis 
