﻿BEITISH FOSSIL CEPHALOPODA. 135 



Orthoceras fretum, Blake, PI. XIY. figs. 7, 7a, 7b. 



Type. — The section is very nearly circular, but the diameter in the direction of 

 the slope of the septa is a little longer than the other. The rate of increase 

 is 1 in 10. The body-chamber is short, its length not being greater than \\ times 

 the basal diameter. It has a well-marked constriction below the aperture, which is 

 otherwise simple. The shell has very close upward imbrications, 20 per line, which 

 get broken off in a peculiar, jagged way ; they are nearly direct, but, if anything, 

 slope in a direction opposite to that of the septa. These have not a greater 

 obliquity than 4°, and have a convexity of more than \ the diameter. Their 

 distance is less than \ of the same. The siphuncle is of moderate size, situated on 

 the longer diameter f- across it, towards the side to which the septa slope back. 

 The greatest diameter is \\ inches ; the length is 3 inches. From the Upper 

 Ludlow, Turner's Hill. In the Museum of Practical G-eology. 



General Description. — The other examples referred to this species only doubtfully 

 belong to it. One has a length of 11 inches, and its septa \ the diameter apart 

 with an excentric siphuncle. The others only show similar imbricating transverse 

 lines of growth, very close together, but none show the aperture. 



Relations. — This has been referred to 0. imbricatum, to which in its septal and 

 siphuncular characters it bears great resemblance ; but in length of body-chamber, 

 and in the constriction beneath the aperture, it is clearly distinct, and the ornaments 

 are more conspicuous than in that species. From 0. pomeroense and 0. mochtreense 

 the position and character of the siphuncle, when seen, separate it. It thus appears 

 that fragments of any of these shells would be un distinguishable. 



Distribution. — In the Wenlock Shale of Barrington (1) ? in the Aymestry 

 Limestone of Usk (1) ? in the Upper Ludlow of Ludlow (1), and in the Upper 

 Silurian of Kirkcudbright Bay (2) ? 



Orthoceras Saturni, Barrande, PI. XI. figs. 6, 6a. 



1868. Orthoceras Saturni, Barrande, 'Syst. Silur. de Boheme,' pi. 255, 264, p. 601. 



Type. — The section is at first circular, but becomes elliptic towards the body- 

 chamber ; the ratio of the axes being as 5 to 6, and the longer one transverse. 

 There is a slight curvature towards the apex in the plane of the minor axis. The 

 rate of increase is about 1 in 6 for the major axis, and 1 in 10 for the minor. The 

 body-chamber is twice the length of its mean basal diameter. The aperture is hori- 

 zontal and simple, and there is a feeble constriction below it. The ornaments consist, 

 first, of very irregular transverse lines, which become more and more lamellose with 

 age, and have a curious zigzag course ; next, of feeble irregularly spaced longitudinal 

 lines, which die away towards the larger end ; and lastly, of a number of small pits 



