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‘ORTHID A. 207 
front divided into two rounded convex lobes, diverging more or less, and divided 
posteriorly or during part of their length by a deep median sinus or groove; hinge-area 
narrow. Ventral valve deeper or more convex than the opposite one, flattened at the 
cardinal angles, and divided longitudinally into two convex lobes by a deep sinus; beak 
more or less incurved ; area triangular, of moderate height; fissure open, triangular ; 
surface of both valves marked by longitudinal strize, which radiate from the umbone or 
beak to the margin of each lobe, one or two smaller or shorter ribs being interpolated 
between each two of the principal ones; surface concentrically marked by fine striz ; 
shell structure punctate. In the interior of dorsal valve a small divided cardinal process 
is situated between two rather widely separate, small, curved brachial processes, on the 
outer side of which, close to the hinge-line, are the sockets. A mesial ridge extends 
from under the cardinal process to the front. Adductor or occlusor scars quadruple ; one 
pair on the bottom of the valve on each side, close to the mesial ridge, but very faintly 
marked. In the ventral valve a prominent tooth on each side at the base of the fissure ; 
dental plates curving inwards on each side so as to enclose a small saucer-shaped cavity, 
longitudinally divided by a somewhat flattened mesial ridge. This cavity is occupied by 
the adductor or occlusor muscular scar (in the middle or on the mesial ridge) and laterally 
by the divaricator and ventral adjustor. A wide, flattened, inwardly dentated band 
margins the interior of each valve. 
Length 7, width 5, depth 2 lines, but more often smaller. 
Obs. This remarkable little shell has been frequently described and illustrated, but 
its true generic position had not been ascertained prior to 1847, when the study of some 
well-preserved interiors of both valves enabled me to demonstrate that its true place and 
affinities were with Orthis, and not with Spirifer, Delthyris, Terebratula, or Dicelosia, 
where it had been located by various paleontologists. In 1767 Linneus describes 
his shell as follows :—‘ Anomia biloba. A. testa biloba equali striata. Hab. ..... 
a. D. Pennant fossilis ;’ but he does not mention any locality or give any reference to 
a figure. In 1847 Messrs. Sharpe, Salter, and myself ascertained that the Linnean col- 
lection contains the shell under description so labelled by Linnzus himself ;* but, sin- 
gularly enough, the Swedes Hisinger and Dalman appear not to have known the shell so 
named by Linnzus, since they give to it the new designation of cardiospermiformis. In 
1815 the species was well figured by James Sowerby, under the new name Zerebratula 
sinuata ; and it subsequently received that of varica from Conrad. In 1850 Prof. 
King proposed for its reception a new genus, Dicelosia, but did not give any valid 
reasons for so doing. As I have shown, all its interior characters are those of Orthis, and 
as such it has been recognised by almost every palzontologist subsequent to 1848. 
Orthis biloba varies very much in shape, and chiefly so by the greater or lesser exten- 
sion and divergence of the free portion of its lobes, the frontal indentation being very 
small in some examples and very deep in others. M. de Verneuil and Dr. Lindstrém 
' See*Hanley, ‘Ipsa Linnei Conchylia,’ p. 134, 1855. 
