LITHODOiMUS. 73 



muscle-scar from the margin, the absence of any byssal sulcus, and the possession 

 of fine radiating stri«. 



Genus Lithodomus, Cuvier, 1817. 



MoDiOLA (pars), Phillips, 1836. Geol. Yorkshire, vol. ii, p. 209. 



— — Morris, 1843. Catal. British Fossils, p. 91. 



— — M'Coy, 1844. Synopsis Carb. Foss. Ireland, p. 74. 

 Lithodomus, M'Coy, 1844. Ibid., p. 75. 



MxTiLXTS, d'Orhiijny, 1851. Prodrome Paleontologie, vol. i, p. 134. 

 Nou Lithodomus, M'Coy, 1855. Class. Brit. Palaeoz. Eocks, p. 493. 

 MoDioLA, Wardle, 1863. Geology of Leek, pi. iii, fig. 6, p. 285. 

 LiTHOPHAGA, Meek and Worihen, 1865. Proe. Acad. Nat. Sci. Phihid., p. 245 ; 



and 1868, Geol. iSurv. lUiuois, vol. v, 

 p. 539. 

 MoDiOLA, R. Etheridye,jun., 1875. Geol. Mag., dec. 2, vol. ii, p. 241. 



— de Koninck, 1885. Ann. Musee R. Hist. Nat. Belgique, vol. xi, p. 175. 

 LiTHOPHAGA, Miller, 1889. North American Geology and Palaeontology, p. 486. 



Generic Characters. — Shell equivalve, transversely elongated, subcylindrical, 

 inflated. Anterior extremity rounded ; the posterior rostrated or cuneiform. 

 Cardinal border straight, subparallel to the inferior border. Umbones very 

 anterior, not terminal, inconspicuous. Hinge edentulous. Ligament marginal, 

 thin, subinternal ; muscle-scars very faint. Shell of two layers ; the inner nacreous, 

 the outer tubular. 



Observations. — From the general shape and the position of the ligament I have 

 thought it Avell to separate Phillips's Modiola lingualis from Modiola, and to place it 

 with Lithodomus dactyloides, M'Coy, although there is no evidence that either 

 species were rock-borers. Messrs. Meek and Worthen were evidently of the same 

 opinion Avhen they named a shell which they thought might be identical with 

 M. lingualis, Phillips, " Lithophaga." 



Fischer, in his 'Manuel de Conchyliologie,' gives Lithophaga (Bolten, 1798) and 

 Lithophagus (Megale von Miihlfeldt, 1811) as synonyms of the Lithodomus of Cuvier, 

 which was invented in 1817. Mr. E. Smith, of the British Museum, informs me 

 that the name was only given in a sale catalogue. 



The elongate, narrow, subcylindrical character of the shell has a far greater 

 resemblance to recent Lithodomi than to Modiola. In Avell-preserved specimens the 

 fine radiating sinuous markings to be seen on the anterior and lower part of the 

 shell of Lithodomus are visible in the Carboniferous forms. This is to be seen in 

 fig. 3, PI. I, and is finely shown in M'Coy's drawing of L. dactyloides. 



The shell described as Lithodomus Jenkinsoni by M'Coy in the ' British 



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