﻿BRITISH FOSSIL CEPHALOPODA. 95 



Orthoceras ibex, Sowerby, PI. V. figs. 3, 3a, 4, 5, 8. 



1838. Orthoceras ibex, Sowerby in Murchison's ' Silurian System,' pi. 5, fig. 30, p. 613. 

 1867. „ „ Dixon, ' Woolhope Nat. F. Club, Fossil Sketches,' No. 1, fig. 2. 



Syn. 1838. Orthoceras articulatum, Sowerby, loc. cit., pi. 5, fig. 31, p. 613. 



1837. „ annulatum, Hisinger, 'Lett. Snecica,' pi. 9, fig. 8, p. 29 (not of Sowerby). 



1857. „ Hisingeri, Boll, ' Arcbiv fur Mecklenburg,' vol. xi. pi. 5, fig. 13, p. 18. 



1870. „ „ Barrande, ' Syst. Silur. de Boheme,' pi. 441, figs. 17-19, p. 700. 



Not 1852. Orthoceras ibex, M'Coy, ' Pal. Fossils,' p. 319. 



1852. „ „ Salter, ' Pal. Fossils,' App. A, p. vii. 



1873. „ „ Salter, ' Camb. and Sil. Fossils,' p. 71, &c. 



1838. Lituites ibex, Sowerby, loc. cit., pi. 11, fig. 6, p. 622. 

 1852. Hortolus ibex, M'Coy, ' Pal. Fossils,' p. 324. 



Type. — This is partly flattened, so that one diameter is nearly double the 

 other ; and the rate of increase is now 1 in 24 for the longer diameter. The ribs 

 are subseparate in character, less than the intervening spaces, but rather rough ; they 

 are slightly undulating, and on the whole 5° oblique, and are distant f- the diameter. 

 The specimen being a cast, the surface ornaments can be but ill made out, but the 

 fact of the original riblets having been transverse is clear. No septal characters 

 are seen. Length, 2 inches ; greatest diameter, -f inch. From the Upper Ludlow, 

 Malvern. In the Museum of the Geological Society. 



General Description. — The minor ornaments being thus transverse in the type, 

 and not longitudinal, as supposed by M'Coy and Salter, we must seek the further 

 elucidation of the species among such as possess this character ; and among them, 

 in fact, we find examples agreeing better with the other features of the type. No 

 examples referable to this species have been seen with more equal diameters than 

 7 and 5 lines, and it may therefore be naturally elliptical. No curvature is seen in 

 any. The rate of increase in the septate portion appears always very small, but the 

 earlier portions of the shell are seldom seen ; the most rapid is 1 in 7, in an example 

 so flattened that one diameter is double the other. In the body-chamber some 

 changes appear to take place. Two examples (PI. Y. figs. 3, 4) which I cannot for 

 any other reason separate from 0. ibex, show a gradual diminution of diameter in 

 the earlier part of the body-chamber, which extends in one of these to a distance of 

 five times its basal diameter, and no aperture is indicated. It is on the strength of 

 this character that 0. Hisingeri has been separated as a distinct species by Boll and 

 Barrande (loc. cit.) ; but it is exceedingly unsatisfactory to have such a character 

 alone as a discriminant. The majority of body-chambers show that at the termina- 

 tion the diameter is increasing, and I think there is proof that the contraction takes 

 place in the middle, and not at the end of the body-chamber (see fig. 4), and these 

 contracting examples belong to its lower half, in which case it must have been of 



