ANIMALS. 183 
more rarely, in the inferior beds at Whitley, Mill-Field Quarry near Bishopwearmouth, 
and in the Breccia at Tynemouth. I have seen a fossil, apparently undistinguishable 
from it, found in the Carboniferous shales near Stanhope, in Weardale. Dr. Geinitz gives 
the following as its German localities: Schwaara, Corbusen, and Umthal, in Zechstein ; 
Kamsdorf, in the Floetz-limestone, between the Weisslegenden and lower Kupfer- 
schiefer beds; Konitz, Muhlberg, Landwehr, Katzenstein, Osterode, and Neuhof, in 
Zechstein-limestone, and Dolomite. I saw, in the mineralogical collection at Kamsdorf, 
a specimen stated to be from Neustadt. It appears to be also characteristic of the 
Permian system in Petschora-land; specimens apparently of the same species have 
been found by Count Keyserling in the Limestone on the Wel near Kischerma, and 
by M. Schrenk in Marl near Ust-Joschuga on the Pinega. 
Family TRIGONIID# (Les Trigonies), Cuvier. 
Adopting the views of Agassiz, the principal characters of Zrigonude consist chiefly 
in the symmetry of the valves, which are regular and perfectly equal, except in the 
parts composing the hinge: their form is sometimes trigonal, that is, the anterior 
extremity is truncated or slightly arched, and the dorsal and ventral margins converge 
backwards in such a manner as to form a rostral prolongation ; sometimes squared, 
that is, truncated posteriorly as well as anteriorly, the upper and under margins being 
nearly straight: the only existing Zrzgonia is nearly orbicular. The shell is inequi- 
lateral, the anterior part being much shorter than the posterior, which is prolonged 
behind considerably: the umbones in many species are considerably curved inwards, 
and even strongly arched, contrary to what is generally observed in the acephalous 
Testacea. The ligament, which forms a strong rounded projection, is external and 
marginal, attached to the edges of a narrow lunated depression or corselet. behind the 
umbones. The surface of the shell is rarely smooth: its sides are generally ornamented 
with ribs, tubercles, and varices, variously combined; whilst the corselet, which 
is generally of considerable extent, and distinctly separated from the sides, is more or 
less smooth, or presents ornaments which often contrast in a most striking manner 
with the other parts of the shell: the only living species is uniformly ornamented with 
transversely pectinated ribs, similar to those of the greater part of the genus Cardium. 
With reference to its dental system; the typical genus of the family has the right 
valve (vide Pl. XIX, fig. 8, 6) furnished with two very salient compressed divaricating 
ridges, sulcated perpendicularly on both sides: the anterior ridge is prolonged towards 
the anterior margin of the shell (B—4*); whilst the posterior one (4—d*) is directed 
backwards. There is a small furrow before the anterior ridge, and another behind the 
posterior one (B—a”, e*). From the disposition of the ridges and furrows of the right 
valve, there results an inverse arrangement of certain parts of the left or opposite valve 
(A) ; while, at the same time, the disposition of other parts is symmetrical : thus, in the 
centre of the hinge of the latter valve there is a large triangular projection (4—oc), 
