ANIMALS. 155 



thus characterised in the shell next to be noticed, and which is commonly placed in 

 the genus Avicula ; on the contrary, it has a deep angular byssal notch (vide PI. XIII, 

 fig. 7 a), a sub-triangular cartilage pit, and a single subcentral muscular impression 

 (vide PL XIII, fig. 20 a), — characters which appear to approximate the genus, claiming 

 it, to Pectenida. Perhaps the claiming genus is typical of an undescribed family, 

 merging Pedenidce into Aviculidcs. This view is somewhat confirmed by the histology 

 which Dr. Carpenter has described of a probably congeneric species, the Avicula 

 cygnipes, Phillips, in which " we find no trace of either the prismatic cellular substance 

 or the nacre, which are characteristic of Avicula ; but we meet, on the other hand, 

 with the coarsely-corrugated and somewhat tubular structure of the Pectenida.""^ I 

 am not acquainted with the character of the cartilage-pit, and the muscular impressions 

 of the shells, which Bronn, Miinster, and Goldfuss have placed in Monotis ; but as 

 their external form closely resembles that of the fossils about to be introduced, lam 

 induced to place it in this genus. 



Monotis speluncaria, ScMotJieim. Plate XIII, figs. 5 to 21. 



Gryphites speluncaeius, Schl. Akad. Miinch., p. 30, pi. v, fig. 1 a, b, c, 1816. 



— — „ Petrefactenkunde, p. 292, 1820. 



— — „ Boue, Edin. Phil. Journ., vol. xii, p. 144, 1825. 

 Productus — „ Brongniart, Tab. de Ter., p. 425, 1829. 



Avicula gryph^oides, J. de C. Sow. Trans. Geol. Soc. Lond., 2d series, vol. iii, p. 



119, 1829. 



— — „ DelaBeche, Geol. Man., p. 384, 1831 ; Germ. 



Transl., p. 459, 1832; 3d English ed., p. 

 573, 1833. 

 Producta speluncaria, Sckl. Op. cit. 



— — „ Phillips, Encyc. Met., vol. iv, p. 617, 1834. 

 Avicula geyph^oides, J. de C. Sow. Op. cit., p. 617, pi. iii, fig- 6 a, b. 



— speluncaria, Sehl. Quenstedt, Wiegmann's Archiv, p. 82, pi. i, fig. 1, vol. 



i, 1835. 

 Peoducta — „ Thomson, Outlines Min. Geol. &c., vol. ii, p. 326, 1836. 



Avicula geyph^oides, J. de C. Sow. Op. cit., p. 318. 



— — „ Phillips, Lardner'sCycL, Geology, vol. i, p. 190, 



fig. 3, 1837. 



— SPELUNCARIA, Schl. Geinitz, Leonhard u. Bronn's Neues Jahrbuch, p. 639, 



1841. 



— — „ Morris, Catalogue, p. 107, 1843. 



— — ,, Geinitz, Gsea von Sachsen, p. 96, 1843. 



1 Report of the Fourteenth Meeting of the British Association, p. 20, J 844. A specimen of an alhed 

 species, Avicula lonyieostata, Stutchbury, in the Bristol Museum, possesses a single sub-central muscular 

 impression, as in Monotis speluncaria. 



