BIVALVIA. 43 



LiTHODOMUS iNCLUsus, Phil. Tab. IV, fig. 13, 13«. 



MoDiOLA INCLUSA. Phil. Gcol. York., i, t. 3, fig. 20, 1835. 



— — Beslongchamfs. Mem. Soc. Linn, de Normand, 1838, pi. 9, 



f. 39, 40. 



Testa parvd, subellipticd, eonvexd, umhonihus subanticis, marline jQosiico compresso, 

 produdo et curvato, striis concentricis tenuissimis irregularibus, lineis incrementi paucis, 

 distantibics. 



Shell small, delicate, subelliptical, tumid, umbones anterior, nearly terminal, posterior 

 margin compressed, produced, and curved ; concentric striae fine and irregular ; lines of 

 growth few, and distant. 



This delicate little shell occurs in all the shelly beds of the Great Oolite, more 

 especially in the beds of soft Oolite which underlies the planking, where the numerous 

 cylindrical crypts sometimes contain it; the cavities themselves filled with calcareous 

 spar, elongated and pyriform, are not uncommon ; the general figure is much more 

 tumid than is observed in the Lithodomi generally. The diameter through both the 

 valves somewhat exceeds the height, and is equal to two thirds of the entire length, which 

 latter rarely exceeds six lines. 



Localities. Minchinhampton Common ; Bisley Common ; Anchff, Wiltshire. It 

 occurs also in the Coralline OoUte of Yorkshire (PhiUips). 



LiTHODOMus PARASITICUS, Deslonc/ckamps, Sp. Tab. IV, fig. 15, 15 «. 



MoDioLA PARASITICA, Desl. Mem. Soc. Linn, de Normand., 1838, t. 9, f. 44 — 46, 

 LiTHODOMUS PARASITICUS, If Orbigny . Prod. Paleont, p. 312, 1850. 



Testa parvd, tenui, obliqud, costis radiantibus magnis paucisque ornatd, interstiis 

 angustis, umbonibus terminalibus acutis. 



Shell small, thin, obUque, lengthened, with terminal acute smooth umbones, the middle 

 and posterior portions of the shell are ornamented with a few large radiating costae, the 

 interstitial spaces narrow and deeply depressed ; the absence of decussating plications, and 

 the acute apex readily distinguishes it from Mytilus pulcherrvmus, the only contempo- 

 raneous allied species. It has occurred very rarely both in the shelly Great Oolite and 

 Stonesfield Slate. 



Localities. Minchinhampton ; Langrune, Normandy. 



Professor E. Deslongchamps records a curious fact connected with the occurrence of 

 this species in Normandy : in a block of stone containing about twenty individuals, each 

 of them occur within the valves of another species, the Modiola {Lithodomtis) inclusa, 

 which had previously effected their perforations in the Umestone. The Rev. H. Jelly 

 has described a somewhat analogous case, as occurring in the Bath Oolite, in which two 

 or three individuals of a species of Modiola lie encased in the valves of a Lithodomus, that 

 had perforated a coral. 



