﻿84 BEITISH FOSSIL CEPHALOPODA. 



comparison, and the likeness is reduced to the slowness of the tapering and the 

 obliquity of the ribs. From 0. inendax and 0. arcuoliratum, the great obliquity of 

 the septa and marginal position of the siphuncle, two characters which usually go 

 together, separate it. 



Distribution. — In the "Lower Llandeilo" Limestone of Durness (6); hence the 

 name. 



Orthoceras arcuoliratum (?), Hall, PI. III. figs. 14, 14a. 



1848. Orthoceras arcuoliratum, Hall, ' Palaeontology of New York,' t. 42, fig. 7, p. 198. 

 1852. „ „ M'Coy, ' Palaeozoic Fossils,' p. 319. 



1859. „ „ Salter, ' Quart. Journ. Geol. Soc.,' vol. xv. p. 375. 



1873. „ „ Salter, ' Cambrian and Silurian Fossils,' p. 71. 



Type. — The section is circular, and the rate of tapering in the figured specimens 

 is 1 in 10. The ornaments are " strong and extremely arching or undulating annu- 

 lations, about equal to the spaces between, and obscure fine longitudinal striae." The 

 septa are not described, but the figure shows them to have a considerable convexity 

 and an obliquity of about 10°, but their distance is unknown. The siphuncle is 

 central and moderate in size. Diameter \ inch. From the Trenton Limestone. 



General Description. — The most probably identified British specimen known is 

 that referred by M'Coy to the above, and which agrees with it so far as its 

 characters are preserved. The section is nearly circular, and the rate of increase is 

 1 in 6Jf. The ribs are obtuse, slightly imbricating forwards, and show something of 

 the strangulated appearance noticeable in Hall's figure ; they are 8° oblique, and are 

 distant f the diameter. M'Coy noticed longitudinal striae on the specimen, which 

 I could not find. Siphuncle central. 



Relations. — It will be seen that this specimen increases at a more rapid rate than 

 the type, and the septa are unknown ; it may, therefore, belong to a different species. 

 It may be the same as Salter's, whose siphuncle is not seen, but 0. arcuoliratum 

 differs from 0. durinum by having that organ central. 



Distribution. — In the Middle Bala, Wrae, Broughton (1) ; in the" Wood wardian 

 Museum, and possibly in the Durness Limestone (1). 



Orthoceras perannulatum, Portlock, PI. III. fig. 4. 



1843. Orthoceras perannulatum, Portlock, 'Geol. Eeport,' t. 25, figs. 5, 6, p. 367. 

 1866. „ encrinale, Salter, 'Mem. Geol. Surv.,' vol. iii. pi. ii. b, fig. 20, p. 356. 



1872. „ „ Salter in Murchison's ' Siluria,' Foss. gr. 9, fig. 10, p. 48. 



Type. — One large and one small specimen were described by Portlock. Neither 

 gives the shapes of section or true tapering, as they consist of surface-markings only. 

 The ribs are of a rounded separate character, as preserved, and are more clearly 



