156 PERMIAN FOSSILS. 
AVICULA SPELUNCARIA, Schl. De Verneuil, Bull. Soc. Géol. de France, 2™° série, vol. i, 
p. 33, 1844. 
— — » Geol. Russ., vol. i, p. 224, 1845. 
», Keyserling, Petschora-land, p. 248, 1846. 
== = » Tennant, Strat. List, p. 88, 1847. 
Monoris — » King, Catalogue, p. 9, 1848. . 
— GRYPHMOIDES, J. de C. Sow. Howse, Trans. T. N. F. C., vol. i, p. 249, 1848. 
AVICULA SPELUNCARIA, Schl. Geinitz, Versteinerungen, p. 10, pl. iv, figs. 18, 19, 1848. 
— KazanEnsis, De Verneuil. Op. cit., p. 10, pl. iv, figs. 20, 21. 
Diagnosis.—Sub-hemispherical; one inch and a quarter in diameter; slightly 
inequilateral, the posterior side being the largest; with a short straight hinge-line. 
Upper valve lobed on its posterior side; with numerous radiating fine, and a few 
strong, ribs; and rather prominent lamelle of growth, the latter often rising vaultedly 
from under each other on the strongest ribs: wmbone rounded, incurved, and projecting 
behind the hinge. Smal/ valve disciform ; finely radiately striated: dyssal notch deep ; 
passing nearly to the centre of the hinge. 
Monotis speluncaria is a variable species in many respects: in general the ribs 
curve towards the posterior side of the shell (vide PI. XIII, fig. 5) ; but in some specimens 
they display a tendency to curve in the contrary direction (vide Pl. XIII, fig. 13). In 
the specimen last cited the large valve is oblique towards the anterior direction ; 
whereas, in general, the obliquity is posteriorly directed: the specimen represented in 
Pl. XIII, fig. 12, is strongly obliquated in the usual direction. In many specimens the 
ribs are of different sizes, every third or fourth rib being larger than the three or four 
intermediate ones: the fossils represented in PI. XIII, figs. 13, 17, are instructive 
examples in this respect, and show that the specimens identified by Dr. Geinitz (vide 
Versteinerungen, pl. iv, figs. 20, 21) with De Verneuil’s Avicula Kazanensis, are merely 
varieties of the present species. The lamellz of growth, as they pass over the large 
ribs, are generally vaulted (vide Pl. XIII, fig. 5); but in some specimens, especially 
those occurring at Silksworth (vide Pl. XIII, fig. 21), they become tubular. Oc- 
casionally a variety is found entirely without ribs, as represented in Pl. XIII, fig. 11. 
My largest specimen, the one represented in Pl. XIII, fig. 20, and exhibiting the 
adductor, pedal, and pallial muscular impressions, measures an inch and a half in diameter. 
M. de Verneuil has described a Russian Permian shell under the name of Avicula 
Kazanensis, which has some resemblance to the present species, but differs from it in 
having a long straight hinge ; in bemg more produced in the cardino-frontal region, 
and more decidedly obliquated. The three specimens figured in the ‘Geology of 
Russia,’ pl. xx, figs. 14 a, 6, c, do not all appear to belong to 4. Kazanensis, inasmuch 
as fig. 14 ¢ (which represents the flat valve), from its slight obliquity, and its radiating 
lmes, might readily be taken for a Monotis speluncaria, were it not that its notch is 
far from being so deep as it is im the latter. 
Monotis speluncaria is a common fossil, both in Germany and England. Von 
