A.NIMALS. 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 Ilmthal, in Zechstein ; 

 Kamsdorf, in the Floetz-limestone, between the Weissliegenden and lower Kupfer- 

 schiefer beds ; Konitz, Miihlberg, 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 Wei near Kischerma, and 

 by M. Schrenk in Marl near Ust-Joschuga on the Pinega. 



Family TRiGONiiDiE [Les Trigonies), Cuvier. 



Adopting the views of Agassiz, the principal characters of Trigoniidce 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 Trigonia 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, B) 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 — 3*) ; whilst the posterior one {B — 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 {A — c), 



