﻿BRITISH FOSSIL CEPHALOPODA. 161 



longer basal diameter, and slightly expands at the aperture, which is somewhat 

 oblique. The septa are always strongly undulating on the broad side. Their con- 

 vexity is about equal to their distance, which is from more than |to } the long- 

 diameter. The siph uncle is on the short diameter from -| to f- of that line from the 

 side. It is seen in three examples to be ^ the diameter, and is probably bulbous. The 

 greatest diameter seen is 2 inches ; the greatest length, that of the type, 5 inches. 



Relations. — This species differs from 0. haculiforme in its siphuncle being on 

 the short diameter, instead of being central or on the long diameter ; from 0. ludense 

 by its undulating septa. This also distinguishes it, together with its slower rate 

 of increase, from 0. subgregarium. It is possible that the examples referred to 

 0. cochleatum from the Tilestones belong to this, though they agree better with the 

 other in their closeness of septa and their rate of increase. 



Distribution. — Examples have hitherto been only seen from the Upper Ludlow 

 of Benson Knot (6). 



Subgenus Actinoceras. 

 Orthoceras (Actinoceras) cochleatum, Schlottheim, PI. XV. figs. 7, 8. 



1813. Orthoceratites cochleatus, Schlottheim, 'Min. Tasch.' vol. vii. p. 34. 

 1820. „ „ Schlottheim, ' Petrefactenkunde,' p. 55. 



1866. Orthoceras cochleatum, Barrande, ' Syst. Sil. de Boheme,' pi. 233-7. 

 Syn. 1821. Orthoceras crassiventris, Wahlenberg, ' Nov. Act. Soc. Sc. Upsala,' vol. viii. p. 90. 

 1837. „ „ Hisinger, ' Leth. Suecica,' pi. 10, p. 30. 



1839. Orthoceras nummularitjm, Sowerby in Murchison's ' Silur. Syst.' pi. 13, fig. 24. 

 1857. „ „ Salter in Murchison's ' Siluria,' pi. 26, fig. 5. 



Type. — Schlottheim merely describes his species as one which has swollen bulbs 

 for its siphuncle, and which occurs in Sweden. The name, however, has been 

 generally adopted and applied to the species described at a later date by 

 Wahlenberg under the name 0. crassiventris. 



General Description. — The British specimens usually referred to this species 

 consist merely of isolated siphuncles, which may of course belong to various species, 

 but there are some at least which cannot be fairly located with any other known 

 species than the present one, and others are placed with these for want of means 

 of distinction. In no example is the true form of section seen. The rate of increase 

 of the sections exposed is 1 in 10 or, at most, 1 in 8. There are no signs of any 

 ornaments on the surface. The septa are on the whole direct, but undulate a little ; 

 their convexity is nearly \ the diameter, and their distance from \ to \ of the same. 

 Sowerby's figured example of O. nummularium shows a distance of the septa of only 

 ^ the diameter, but those seen may be the last two of the series, which are often 

 closer. The siphuncle appears to vary in position according to the preservation of 



