ANIMALS. 73 
of corresponding size of the same species exhibit any trace of this structure.’ As 
already observed, the foraminated deltidium of Orthisina adscendens does not appear to 
be general in the species. It will thus be understood, that Iam little disposed to agree 
with M. Alcide d’Orbigny’ in making the foramen in Strophomenide a generic character. 
Most of the palliobranchiate genera are furnished with a prominency, more or less 
developed, in the centre of the hinge of the small or imperforate valve (Pl. VI, fig. 
And PIOVIT, fies. 3) 5d;) Pla XI figs: 10¢,.11,°12; Pl. XIX, fig: 3a; Pl. XX, 
fig. 7e). This 4oss, as in future it will be termed, exists under various forms in 
different genera; it is very small in Clecothyris pectinifera (Pl. X, fig. 9c); rounded, 
and a trifle larger in Camerophoria multiplicata; about the same size, and cup-shaped 
in Valdheimia Australis (Pl. XX, fig. 11 J); larger, and bisected in Leptena analoga 
(Pl. XX, fig. 7 e); still larger, and bi- or trilobed in Productus horridus, (vide Pl. XI, 
figs. 11, 12), and some other genera; elongated, and somewhat erect in Orthis eaimia, 
and some species belonging to the Cretaceous system; and drawn out, assuming a 
nearly horizontal position on a thick cardinal plate in Bouchardia rosea. In 
Trigonotreta undulata (Pl. 1X, fig. 7 6), Hypothyris psittacea, and many other shells, 
the boss does not exist; while, on the other hand, it is enormously developed in 
Strigocephalus Burtint (Pl. XIX, fig. 1 e), forming a massive, curving process, stretching 
from the hinge to nearly the centre of the opposite or large valve, where it clasps, as it 
were, the ventral median plate by means of its dilated bifurcated extremity. These 
modifications will show that the boss has not served as a tooth or an articulating instru- 
ment, as is generally considered ; the cavities (sockets) on each side of it, and their 
occupying parts, were the only structures adapted for this purpose. As will after- 
wards be shown, the boss served as a fulcrum for certain muscles; but it may be 
observed, in the present place, that its non-articulating character is clearly proved by 
the markings usually displayed on its surface in Productus (Pl. XI, fig. 11) and Leptena 
(Pl. XX, fig. 7 e), and by the complete absence in the hinge of the opposite valve of a 
correspondingly marked depression in which it could act. 
The muscular system of the Palliobranchs may now be described. With perhaps 
but few exceptions, the umbonal cavity of a brachiopod shell is furnished with a 
dense, fibrous, cylindrical body, termed the pedicle. In Waldheimia Australis the inferior 
end of the pedicle fits into the foramen (Pl. XX, fig. 12a), and its superior end, 
which is somewhat flattened or dilated in the transverse direction of the shell, is 
situated at the entrance or anterior part of the rostral cavity, to the surface of which 
it appears to be attached by means of tendinous or membranous chords; the truncated 
extremity of the pedicle itself apparently not being adherent. There is one genus, 
1 Mr. D. Sharpe has founda “foramen in some young specimens” of the same shell from the Wenlock 
Slate. (Vide Quarterly Journal of the Geol. Soc., vol. iv, p. 172.) 
2 Comptes-rendus de l’Acad. des Sc., vol. xxxvi, 5 Aoat, 1847. 
3 Geology of Russia, vol. 1, p. 192, pl. xi, fig. 2 e. 
