﻿BRITISH FOSSIL CEPHALOPODA. 113 



greater in youth ; the mean distance is about ^ of the compressed diameter, but the 

 last few are crowded together. The siphuncle is seldom seen ; in a small example it 

 is f the short diameter from the concave side, in a larger one it is ^ the long diameter 

 from the side, in others it is central, showing a variation with age as in the type. 

 The form of its elements is seen in only one, in which it slightly expands between 

 the septa and becomes obliquely strangulated by them (fig. 6). The largest specimen 

 seen had a length of 13 inches and a diameter of 4 inches, but fragments indicate a 

 still larger possible size ; usual specimens are from 4 to 8 inches in length. 



Relations. — This species has usually passed in collections either for 0. angulatum 

 or O.Jilosum, but has occasionally been indicated as a new form. From 0. angulatum 

 its difference is very marked ; that species has no intermediate riblets of any kind. 

 When, however, both are in the state of casts, this distinction fails ; but the whole 

 ornaments of 0. Bacchus being more feeble, only produce at most a polygonal cast 

 with rounded angles, instead of an acutely marked one. The present species is a 

 much larger one. From 0. jilosum it differs in the unequal development of its 

 riblets and the larger size of the primaries ; but when in the body-chamber, the 

 distinctions fade away: the difference of some varieties at least is very slight; and 

 as 0. Jilosum was named from a large body-chamber, the existence of that species as 

 distinct from an adult 0. Bacchus has to be proved, which proof will be found under 

 the description of the species. Another form nearly allied is 0. coralliforme, which 

 very closely represents the young, and some examples are very difficult to distinguish. 

 When, however, the ornaments are well preserved, they are seen to be different, the 

 longitudinal lines are more elevated, and the transverse ones are equal in number to 

 them. The earlier 0. coralliforme is doubtless the progenitor of 0. Bacchus, and in 

 this respect is instructive as representing its early stage. Yet the modifications are 

 sufficient to justify a separate name for the giant Orthoceras of the later period. 



Distribution. — The earliest representative is a doubtful one from the Upper 

 Llandovery of Myddfai, in the Museum of Practical Geology (1). It occurs certainly 

 in the Wenlock Shale of Usk (3) ; in the Wenlock Limestone, Donnington ; in the 

 Lower Ludlow of Ledbury (9, mostly in the collection of Dr. G-rindrod), Mock- 

 tree (1), and Dudley (1) ; in the Aymestry Limestone, and Upper Ludlow (3) at 

 Ludlow ; in the Upper Silurian of Kerry (4) and of the Pentland Hills (4). 



Orthoceras filosum, Sowerby, PI. X. figs. 8, 8a, 9. 



1838. Orthoceras filosum, Sowerby in Murchison's 'Silurian System,' tab. 9, fig. 3, 



p. 620. 

 1852. „ „ M'Coy, 'Palaeozoic Fossils,' p. 314. 



1873. „ „ Salter, ' Cambrian and Silurian Fossils,' p. 173. 



Type. — I have been unable to find this in the Collection of the Geological Society, 



