﻿164 BRITISH FOSSIL CEPHALOPODA. 



Relations. — This species is of course closely allied to E. duplex, but it never- 

 theless seems distinct ; for its rate of increase is 3 times as great, its section is 

 naturally elliptic, and the septa are more than twice as approximate. From 

 E. Brongniartii its more lateral siphuncle distinguishes it. If the " Endoceras" 

 vertebrale, Eichwald (' Leth. Ross.' pL 46, fig. 4)> is rightly drawn, it must belong 

 rather to the subgenus Conoceras ; but if it be really an Endoceras, this species will 

 be very close to it, the only difference being that our species has less distant septa. 



Distribution. — In Lower Silurian Beds, Worthen, Shropshire (1). The specimen 

 is in the British Museum. 



Orthoceeas (Endoceras) vaginatum, Schlottheim. 



1813. Orthoceratites vaginatus, Schlottheim, ' Min. Taschenbuch,' vol. vii. p. 69. 

 1845. „ „ De Verneuil, ' Geology of Kussia,' vol. ii. pi. 24, fig. 6. 



1851. Orthoceras vaginatum, Salter, ' Quart. Journ. Geol. Soc.' vol. vii. pi. 10, fig. 7. 



General Description. — I have not seen the example referred to by Salter, nor any 

 other which might represent this species, yet his figure leaves little doubt of its 

 really occurring at Girvan. No section is visible ; but the rate of increase of the 

 diameter exposed is about 1 in 11. There are well-marked transverse direct riblets 

 and stronger undulations at a distance of ^ the diameter. The septa mark the 

 siphuncle with sheaths as in E. duplex, which, though in the figure they look to 

 point forwards, are doubtless nearly direct ; they are distant ^ the mean diameter, 

 and the siphuncle itself is § of the same. The greatest length is 2f inches, and 

 greatest diameter 1£ inches. 



Relations. — The transverse ribs separate this species from all other Endocerata 

 that occur in British rocks ; and there is nothing in this specimen that should 

 separate it from the Russian species. 



Distribution. — In the Middle Silurian of the Girvan district. 



Subgenus Tretoceras. 

 Orthoceras (Tretoceras) bisiphonatum, Sowerby, PL XYI. figs. 3, 3a, 3 J, 4. 



1839. Orthoceras bisiphonatum, Sowerby in Murchison's ' Silurian Syst.' pi. 21, fig. 23. 



1858. Tretoceras bisiphonatum, Salter, ' Quart. Journ. Geol. Soc' vol. xiv. pi. 12, p. 177. 

 1874. „ „ Barrande, ' Syst. Silur. de Boheine,' vol. ii. p^ 800. 



Query 1868. Actinoceras baccatum, Woodward, ' Geol. Mag.' vol. v. pi. viii. p. 133. 



Type. — There are two fragments associated with the piece figured by Sowerby 

 belonging to the same specimen. The section is not actually visible, but it appears 

 to be circular (fig. 3a). The rate of increase is very small. The body-chamber is 

 prolonged backwards through the septa, preserved in a tube of f in. diameter ; this 



