﻿230 BRITISH FOSSIL CEPHALOPODA. 



Genus Ophidioceras. 

 Ophidioceras articulatum, Sowerby, PI. XYIII. figs. 14, 14a, 15. 



1838. Lituites articulatus, Sowerby in Murchison's 'Sil. Syst.' pi. 11, fig. 5 (not fig. 7), p. 622. 

 1873. „ „ Salter, ' Cambrian and Silurian Foss.' p. 174. 



Not Lituites articulatus, M'Coy, ' Palasozoic Fossils,' p. 323. 



Type. — Mean rate of increase 1.31. Last whorl .27 of the whole, the several 

 whorls being just in contact. There are indications of the last whorl leaving the 

 rest by the diminution of curvature. The shape of the section is unknown, but 

 the outside looks as if it were keeled. The nearly straight ribs are narrow and 

 separate, and have a sigmoidal bend outside. No septa or siphuncle seen. Diameter 

 I5 inches. From the Lower Ludlow of Elton, near Ludlow. In the Museum of the 

 Geological Society. 



General Description. — The rate of increase, as measured, ranges from 1.43 to 

 1.29 ; but from the compression to which the specimens have been subjected, these 

 measures are seldom very reliable. The inner whorls are always just in contact — 

 till the body-chamber, which leaves the coiled part at a diameter from 9 to 14 lines, 

 and continues straight for § the greatest diameter. In an obscure fossil referred to 

 this species from the Wenlock Shale, the uncoiled part reaches twice this distance. 

 There is no sign in any of the slightest want of symmetry. The ribs are always 

 direct, more or less separate, with a slightly backward direction towards the outside, 

 from 26 to 28 per whorl. The front had a flattened band along it as shown by two 

 smooth parallel lines in more than one example. The finer ornaments are parallel 

 lines of growth, and the surface is occasionally pitted (fig. 4a). The section when 

 unflattened is uniformly rounded. The body-chamber consists of some part of the 

 coiled portion ; the ribs die out towards the aperture, which is not seen to be con- 

 tracted. The septa are more remote than the ribs, being but 14 per whorl in the 

 earlier part. The siphuncle is only seen in one example in the Wenlock Limestone, 

 where it is nearly external, being preserved after the decay of the shell. The 

 diameter is never more than 1^ inches. 



Relations. — The straight ribbing and the band along the front easily distin- 

 guish this from previously described British forms ; but it is very similar to some 

 of the other Ophidiocerata figured by Barrande on pi. 45 of his Silurian Cephalo- 

 pods, with none of which, however, it exactly agrees, but is nearest to 0. tenerum, or 

 0. simplex. The contracted aperture has not, however, been seen in British examples. 



Distribution. — There are but few good examples of this rather rare species. It 

 occurs in the Wenlock Shale at Oernant (1), in the Wenlock Limestone of Wen- 



