T. Davidson — What is a Brachiopod ? 
201 
shall adopt, as they seem to carry with them a greater degree of 
plausibility* (PL X. Figs. 1, 2). Of the shell or valvular muscles he 
makes out five pairs and an odd one, and individualises tkeir 
respective functions as follows. Tkree pairs are laterals, having 
their members limited to the sides of the shell : one pair are 
transmedians, each member passing across the middle to the 
reverse side of the shell : while the odd muscle occupies the um- 
bonal cavity. The central and umbonal muscles effect the direct 
opening and closing of the shell : the laterals enable the valves to 
move forward and backward on each other : and the transmedians 
allow the similar extremities (the rostral) of the valves to turn from 
each other, to the right or the left, on an axis subcentrically situated, 
that is, in the medio-transverse region of the dorsal valve. It was 
long a matter in discussion whether the animal could displace its 
valves sideways when about to open its shell ; but this point has 
been set at rest by Professors Semper and Morse, who observed the 
animal perform the Operation. They mention that it is never done 
suddenly, or by jerks : as the valves are at first always pushed to 
one side several tirues, and back again on each other, at the same 
time opening gradually in the transverse direction, tili they rest 
opposite to one another and widely apart. Those who had not seen 
the animal in life, or who did not believe in the possibility of the 
valves Crossing each other with a slight obliquity, would not consent 
to appropriating any of its muscles to that purpose, and consequently 
attributed to all the lateral muscles the simple function of keeping 
the valves in an opposite position, or liolding them adjusted. We 
have not only the observations of Semper and Morse, but the anato- 
mical investigations of King to confirm the sliding action or lateral 
divarication of the valves of Lingula. 
In the Clistenterata, where no such sliding action of the valves was 
necessary, or possible, no muscles for such au object were required : 
consequently none took rise from the lateral portions of the valves, as 
in Lingula. In an extinct group, however, the Trimerellidce, which 
seems to be somewhat intermediate in character between the Treten- 
terata and Clistenterata, certain scars have been found which appear 
to have been produced by rudimentary lateral muscles; but it is 
donbtful (considering the shells are furnished with teeth, though but 
rudely developed) that such muscles enabled the valves, as in Lin- 
gula, to move forward and backwards upon each other. There are 
muscles connected with other portions of the animal, such as the 
parietal so strongly defined in the Tretenterata, and distinctive pecu- 
liarities of the peduncle, etc.; but the limited space at my disposal 
will not admit of more than a passing allusion to them. 1 2 
Banges of Depth. — All Brachiopoda are inhabitants of the sea. A 
vast amount of important and accurate information has been collected 
during the past ten years with respect to the geographical distri- 
1 King, Annals and Mag. of Nat. Hist., 4th series, vol. xii., 1873. 
2 Davidson and King, on the Trimerellidce, Quarterly Journal of the Geological 
Society, 1874, vol. xxx. p. 124. 
