156 PERMIAN FOSSILS. 



AvicuLA SPELUNCARIA, ScJil. De Vemeuil, Bull. Soc. Geol. de France, 2°" serie, vol. i, 



p. 33, 1844. 

 , Geol. Russ., vol. i, p. 224, 1845. 



Keyserling, Petschora-land, p. 248, 1846. 

 Tennant, Strat. List, p. 88, 1847. 

 King, Catalogue, p. 9, 1848, 

 de C. Sow. Howse, Trans. T. N. F. G., vol. i, p. 249, 1848. 

 AvicuLA SPELUNCARIA, Schl. Geinitz, Versteinerungen, p. 10, pi. iv, figs. 18, 19, 1848. 

 — Kazanensis, De Vemeuil. Op. cit., p. 10, pi. iv, figs. 20, 21. 



MONOTIS 



— GRYPHjEOIDES, J. 



Diagnosis. — Sub-hemispherical ; one inch and a quarter in diameter ; shghtly 

 inequilateral, the posterior side being the largest ; with a short straight hinge-line. 

 Tipper valve lobed on its posterior side ; with numerous radiating fine, and a few 

 strong, ribs ; and rather prominent lamellae of growth, the latter often rising vaultedly 

 from under each other on the strongest ribs : umbone rounded, incurved, and projecting 

 behind the hinge. Small valve disciform ; finely radiately striated : hyssal 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 PI. 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 

 PI. 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, pi. iv, figs. 20, 21) with De Verneuil's Avicula Kazanensis, are merely 

 varieties of the present species. The lamellae 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 PI. XIII, fig. 21), they become tubular. Oc- 

 casionally a variety is found entirely without ribs, as represented in PI. XIII, fig. 1 1 , 

 My largest specimen, the one represented in PI. 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 being more produced in the cardino-frontal region, 

 and more decidedly obliquated. The three specimens figured in the ' Geology of 

 Russia,' pi, XX, figs, 14 a, b, c, do not all appear to belong to A. Kazanensis, inasmuch 

 as fig, 14 c (which represents the flat valve), from its slight obliquity, and its radiating 

 lines, might readily be taken for a Monotis speluncaria, were it not that its notch is 

 far from being so deep as it is in the latter. 



Monotis speluncaria is a common fossil, both in Germany and England. Von 



