﻿152 BRITISH FOSSIL CEPHALOPODA. 



upon it. The septa are seen in the figures to pass imperceptibly into the sides of 

 the shell. The siphuncle is small and situated excentrically on the longer diameter. 

 The diameter of these shells varies from -|- inch to 3^ inches, and the greatest 

 length is 6 inches. They occur in a " colony," and in the beds Ei and E2. 



General Description. — The presence of this remarkable species in England is 

 indicated chiefly by a fragment in the Museum of Practical Geology. The diameters 

 are in the ratio of 16 to 15, and the rate of increase is small. The septa are slightly 

 oblique, and show very clearly the peculiar feature of being continuous with the 

 sides. The convexity of their surface is about \ the diameter. The siphuncle is 

 on the long diameter about f its length from the side. The shell appears to be 

 quite smooth. This has a diameter of \\ inches. Besides this are some broken-off 

 septal chambers, presumably due, like those of 0. imbricatum, to natural truncation, 

 whose surface runs imperceptibly into the side. With these may possibly be 

 associated some smaller specimens with very bulbous septa, whose distance is almost 

 ^ the diameter, and siphuncle as in the type. 



Relations. — The peculiar feature of this species is most nearly approached by 

 0. Etheridgii, which, however, is well marked off from it. The species figured by 

 Barrande as 0. Thompsoni has almost confluent septa, but its rate of increase is" 

 much greater. 



Distribution. — In the Woolhope Limestone, Littlehope (1) ; in the Coldwell 

 Flags (4). The smaller specimens are from Garcoed, Usk (3), and the Pentland 

 Hills (2). 



Orthoceras excentricum, Sowerby, PI. XII. figs. 2, 2a, 3. 



1838. Orthoceras excentricum, Sowerby in Murchison's 'Silurian Syst.' t. 13, fig. 16, p. 631. 



Type. — The section is slightly quadrate, but has equal axes ; the rate of increase 

 is 1 in 5. The whole is septate, and where the shell is partly preserved are seen 

 slightly marked longitudinal furrows, less than \ inch apart, leaving convexities 

 between : these, however, are not true ornaments, but merely representatives of 

 multiple normal lines as seen in 0. ludense and others. The septa are a little oblique, 

 and have a very slight convexity, -|- of their diameter; their distance is \ of the 

 same. The siphuncle is f across the diameter towards the side to which the septa slope 

 back, and is itself -^ of the same. Length, 2 § inches ; greatest diameter, 1 \ inches. 

 From the Wenlock Shale, Radnor. In the Museum of the Geological Society. 



General Description. — Other examples confirm most of the characters of the type 

 — namely, the rate of increase, and the extreme flatness of the septa. No signs of 

 ornament are seen in any. The septa are slightly undulating, but their distance is 

 very constant. The siphuncle may have a position as far from the centre as \ of the 

 whole diameter. 



