﻿BRITISH FOSSIL CEPHALOPODA. 103 



Orthoceras dimidiatum, Sowerby, PI. VI. figs. 11, 12. 



1839. Orthoceras dimidiatum, Sowerby in Murchison's ' Silurian System,' pi. 8, fig. 18, 



p. 620. 

 1852. „ „ M'Coy, ' Pal. Foss.' p. 314. 



1852. „ „ Salter, Appendix A to ' Pal. Foss.' 



1873. „ „ Salter, ' Camb. and Sil. Foss.' pp. 98, 173. 



Syn. 1848. Orthoceras subdimidiatum, D'Orbigny, 'Prod.' vol. i. p. vii. 



Not 1841. Orthoceras dimidiatum, Miinster, ' Beit.' vol. iii. t. 19, figs. 3-5. 



Type. — No section is seen. The rate of increase is 1 in 18. No characters of the 

 body-chamber are seen. The ornaments are semi-ribs, found only on the right side 

 of the specimen ; these are slightly undulating, but slightly oblique, rising to the 

 left side ; they are \ the mean diameter apart, and consist of downward imbrications 

 which gradually die away. There may have been finer lines parallel to them. The 

 septa are obscurely seen on the unribbed side, about \ the diameter apart and nearly 

 direct. Length, 2 inches ; diameter, \ inch. From the Low 7 er Ludlow, Radnor 

 Forest. In the Museum of the Geological Society. 



General Description. — The specimens which certainly belong to this species are 

 all either external casts (fig. 10), or surface-markings only (fig. 11). The average 

 rate of increase, as observed, is 1 in 16. The body-chamber is several times the 

 length of its basal diameter. The aperture has a sigmoid outline passing into a 

 forward curve on the unornamented side (see fig. 11). The ornaments can only be 

 described as imbrications passing obliquely half across the shell, as it is impossible 

 to dismember the species according to the minor varieties. With regard to the 

 direction of the imbrication, it may be either upward or downward ; but in two 

 examples, one kind appears to change by degrees into the other. In most, the lower 

 half of the ribs is preserved, but in some it is the upper. In the former case they 

 lie on the ventral side, but in specimens of the latter the aperture has not been 

 observed. These ornaments are generally oblique from 7° to 10°, but may be more 

 so by distortion. Their distance is seen in one example to increase with age, being 

 nearly twice as close at the small end ; on the average they are \ the diameter 

 apart. The septa are parallel to the ribs. The distance is not constant, but 

 varies \ to more than \ the diameter. The siphuncle has not been seen. The 

 greatest length seen is 3 inches ; and the greatest diameter, 4 lines. 



Relations. — Where compressed as usual so as to show the semi-ribs, this species 

 is easily distinguished from all other British species; but it may be occasionally 

 flattened so that the ribbed side only is exposed : it then resembles 0. sitbundulalum, 

 but the imbrications are stronger, more usually downward, and show some signs of 

 decrease. Its representative in Bohemia appears to be 0. bifrons, from which it 

 differs in having stronger ornaments and more remote septa. 



