MODIOLA TRANSVERSA. 67 



sjDeaks of his var. lata having '' a flatteued form, j^robably the result of pressure.' 

 This flattening is only apparent and not real, and is due to the larger size of the 

 shell and the smooth surface of the cast. 



This shell has somewhat the ajjpearance of Anthracomya, from which it can be 

 separated by the absence of the groove parallel to the hinge-line in casts. 



There is also a superficial resemblance to Naiadites, from which it is separated 

 by the absence of the characteristic pit-like, triple, anterior muscle-scars and the 

 striated hinge-plate. 



When young the shells are not so oblique, and are in j)roportion to their size 

 more tumid, and have the anterioi' end more developed relatively to the rest of the 

 shell, PI. II, fig. 24. There is a fine slab from the Portlock Collection in the 

 Geological Survey Museum, Jermyn Street, showing this species in all stages of 

 growth ; even when very young the internal ridge is conspicuous. 



This species seems to be, as far as is at present known, confined to the Lower 

 Limestone-shales of the North of Ireland, and to be associated with Leperditia Okeni, 

 Helodus, and plant remains. It is doubtful whether it is therefore of marine 

 habitat, and in this particular differs very markedly from Modiola Macadamii, which 

 occurs with undoubtedly marine shells {vide antea, p. 64). The lithological character 

 of the shales in which the two forms occur is very different, Modiola lata being 

 found in brittle and soft marly shales, light grey in colour. De Koninck, in the 

 remarks on his shell Modiola macrocephala, ' Faune de Calcaire Carbonifere de la 

 Belgique,' p. 178, says of his species, " EUe a quelque ressemblance avec la Modiola 

 Macadamii, var. lata, de J. E. Portlock, mais elle en difFere par sa grande taille et 

 par sa forte epaisseur." The Belgian shell is so very much larger than the Irish 

 specimen, measuring 70 mm. in length, 43 mm. in height, and 30 mm. in thickness, 

 that I think there can be no connection between the two forms. De Koninck's 

 species is founded on a single specimen from the Limestone of Waulsort. 



Modiola transversa, sp. nov.. Hind. PI. II, figs. 11, 12. 



Specific Characters. — Shell small, transversely compressed, ovate, gibbose. The 

 anterior end is small and obtusely rounded, the narrowest part of the shell dorso- 

 ventrally. The inferior margin, directed backwards and slightly downwards, is 

 straight, joining the anterior and posterior margins at either end with a regular 

 blunt curve. The posterior border is regular and almost semicircularly rounded. 

 The hinge-line is straight, equal in length to about two-thirds of the antero- 

 posterior diameter of the shell. The umbones are small, anterior, pointed, and 

 directed inwards, downwards, and forwards ; not elevated above the hinge-line. 

 Proceeding obliquely do"wn wards and backwards is a blunt gibbosity, not well marked 

 off from the rest of the shell. The swelling is most marked above. About the 



