Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/244

196 190 J. S. GABDKEE ON BRITISH CRETACEOUS PATELLIDiE ETC.

FISSURE LLID^E.

Emarginula neocomiensis, D'Orb. Neocomian. PI. VIII. figs. 1-6.

Shell oblong, twice as long as high, solid, conical, convex ante- riorly, slightly concave posteriorly ; apex small, recurved, about § posterior ; sculpture, 26 to 30 strong narrow principal ribs, and the same number of intermediate ribs, the last varying in prominence in different specimens, and not appearing at all until the shell is well advanced in growth ; the ribs are cancellated in full-grown specimens by 25 to 30 elevated lines of growth, which pass over the secondary, but not over the principal ribs (fig. 6a). In two very perfect specimens in the Leckenby collection, Woodwardian Museum, there is a node at every intersection (figs. 1, 2, 2 a). The slit is short, and placed rather to the right of the median line. D'Orbigny's figure in the ' Pal. Prang.' has this character rather exaggerated ; and it usually occupies a more symmetrical position. Pictet and Cain- piche had five examples under examination, in all of which the fissure deviated. Amongst the number of specimens which I have exa- mined, the fissure, I find, usually deviates ; but the character is not constant. The slit is bordered on each side by a principal rib ; the scar forms, between two of the principal ribs, a narrower region than the space occupied by an intermediate rib, and is imbricated trans- versely. There is no trace of spiral growth in any of the specimens. The cast is smooth, except at the margins, where it bears impres- sions of all the ribs. The line of fissure presents a broad furrow, ex- tending from near the apex to the margin, in the centre of which is a slightly raised region terminating at the fissure (PI. VIII. fig. 2).

Although very distinctly characterized, it has been confounded with E. Guerangeri, D'Orb., of the Chloritic Marl, from which it is readily distinguished by its greater number of ribs.

Pictet and Campiche, 'Terr. Cret. de Ste. -Croix,' mention a va- riety which has the primary and secondary ribs equally prominent ; there is a similar example in the Woodwardian Museum, from Don- nington, Lincolnshire, which has about 54 ribs and a median fissure, and may prove distinct when more specimens are available.

There is apparently no specific difference between the specimen from the Atherfield clay, and one from Speeton, in the Jermyn-street Museum, named E.Jlssaria, Forbes.

The largest specimen I have seen is from Seend, and is in the British Museum (figs. 3, 4), measuring 1. 2b millims., b. 18 mil- lims., ht. 13 millims., these dimensions being considerably in ex- cess of those usually met with.

E. neocomiensis, D'Orb., was first described as E. reticulata by Leymerie, in 1842, and was figured the next year by D'Orbigny in the ' Pal. Franc. Terr. Cret.' vol. ii. p. 392, pi. 234. figs. 4-8, and again by Pictet and Campeche, ' Terr. Cret. de Ste.-Croix,' pi. xcvii. figs. 9-11.

The species is very generally distributed throughout the Lower Green sand, which it characterizes, and has, no doubt, as wide a