﻿BRITISH FOSSIL CEPHALOPODA. 1.51 



up to -j^ when larger. The siphuncle is always central in the sections seen, and one 

 shows it nearly central on the septal surface. The inner and outer coats of the 

 siphuncle are always separated by deposits on their surface, and there are similar 

 deposits in the septa ; the outer coat had folds on its surface ; the whole occupies 

 from -g- to ^ the diameter. The greatest diameter seen is If inches. 



Relations. — The peculiar character of the siphuncle combined with the rapid 

 increase are the distinguishing features of this species. In 0. cochleatum the 

 tapering is not so rapid. 



Distribution. — In the Upper Llandovery of Tortworth (1) and Marloes Bay (1) ; 

 in the Wenlock Shale of Builth (1), and near Ludlow (1) ; in the Upper Ludlow of 

 Usk (3). Possibly also specimens from the Lower Llandovery of Blaen-y-cwm 

 belong to this. 



Orthoceras circulare, Sowerby. 



1814. Orthoceras circulare, Sowerby, ' Min. Con.' pi. 60, figs. 6, 7. 



Type. — I have not seen among Sowerby's fossils any which corresponds to this. 

 His figure shows the section to be elliptic, with the diameters in the ratio of 15 to 14. 

 The rate of increase of the longer diameter is 1 in 6. The whole is a septate cast, 

 the septa very slightly oblique, and distant ^ the long diameter ; the siphuncle is 

 \ along the greater diameter. From the Upper Silurian, Dudley. 



General Description. — One example seen may perhaps represent this. It has a 

 nearly circular section, and its rate of increase is 1 in 5. It possibly had some 

 longitudinal ornaments. The septa are undulating and distant \ the diameter. 

 The siphuncle is situated -^ of the diameter from the side, and appears to have 

 globular elements. The greatest diameter is 8 lines, and the length 1 inch. 



Relations. — This is an exceedingly doubtful species. It differs from 0. conicum, 

 if its siphuncle is truly complicated, in the very excentric position of that organ. It 

 differs from 0. cochleatum in its rapid increase and closer septa. 



Distribution. — In the Upper Llandovery, Bogmine. Sowerby's was from Dudley. 



Orthoceras trtjjstcatum, Barrande, PI. XIY. fig. 8. 



1868. Orthoceras trtjncatum, Barrande, ' Syst. Silur. de Boheme,' pi. 341-343, p. 556. 



Type. — The section is elliptic, the diameters being in the ratio of 5 to 4. The 

 rate of increase is very slow. The body-chamber decreases in size, and the aperture 

 is oblique and undulating. The shell in most is smooth. The septa are nearly 

 direct, and distant about -^ the diameter. Their convexity is very great, but 

 diminishes towards the body-chamber. The several chambers are successively cut 

 off, and a layer of shell is deposited on the truncated end with various markings 



