﻿BRITISH FOSSIL CEPHALOPODA. 137 



Orthoceras mocktreense, Sowerby, PI. XV. figs. 6, 6a, 6b ; 

 also PL X. figs. 2, 6, and PL XI. fig. 3. 



1838. Orthoceras mocktreense, Sowerby in Murchison's 'Silurian System,' pi. 6, fig. 11, p. 606. 



1848. „ „ Phillips and Salter, ' Mem. Geol. Surv.' vol. ii. pt. i. p. 353. 



1852. „ „ M'Coy, ' Palaeozoic Fossils,' p. 315. 



1873. „ „ Salter, 'Cambrian and Silurian Fossils,' p. 158. 



Type. — The section is very nearly circular, the ratio of the diameters being 

 21 to 20. The rate of increase is perhaps 1 in 13. The whole specimen is septate. 

 The ornaments consist, first, of well-marked somewhat undulating fine upward im- 

 brications, like lines of growth : secondly, of very fine lines parallel to these, about 

 30 per line ; over all this is seen a curious network of polygonal cells arranged 

 quincuncially, with their long axes longitudinal, about three per line transversely. 

 This appearance may be due to the former overgrowth of some polyzoon, but it is 

 very uniform on the surface. The lines of growth are irregular in places, from 

 mending. The septal distance can only be judged by the longitudinal diameter of 

 the siphuncular bead ; this gives it as f the diameter. The siphuncle has its centre 

 about -| across the diameter, and is inflated to a bead between the septa, where it is 

 nearly ^ the diameter ; it is therefore rather longer than broad : exteriorly, but 

 perhaps from preservation, it has a mammillated surface. This specimen is very 

 remarkably preserved, one half being dark, the other light, the division between the 

 two colours corresponding to a crystalline plane. Length, 2 inches ; diameter, 

 nearly 1 inch. From the Lower Ludlow of Mocktree. In the Collection of the 

 Geological Society. 



General Description. — The preservation of the type is so peculiar that it is difficult 

 to identify with it specimens which are preserved in the ordinary way. The most 

 important examples are therefore figured (PL X. figs. 2, 6 ; PL XL fig. 3). These 

 are all characterised by a general similarity of ornaments, but the bulbous siphuncle 

 is only seen in other examples. The section is circular, and the rate of increase 

 generally about 1 in 10 in small specimens, decreasing to very little in the body- 

 chamber. The latter, in the large example (PL XL fig. 3), is 3^ times its basal 

 diameter, and is slightly contracted before the aperture is reached. The ornaments 

 are essentially upwardly imbricating lines of growth, with finer intermediate lines, 

 but the polygonal network is not seen again. The septa are on the whole direct, 

 though a little undulating, and are distant \ to f the diameter. Some examples 

 show a bulbous siphuncle f across the diameter. The largest has a diameter of 

 3^ inches, and a length of 12f inches. 



Relations. — The bulbous siphuncle when seen is sufficient to distinguish this 

 species. It is allied, except for this, to 0. Saturni, whose ornaments differ but 



