﻿BEITISH FOSSIL CEPHALOPODA. 149 



not only of their septa, which might approximate them to the genus Hyolithes of 

 Eichwald, but also of their siphuncle. It is, however, difficult to be certain of the 

 original form of Orthoceras that produced them, and it is rather from want of 

 means of separation than from positive proof of identity that the details derived 

 from uncrushed examples are placed to the credit of this species. One section is 

 seen elliptic, with diameters in the ratio of 6 to 5. The apparent rate of increase 

 depends very much upon the amount of compression, and the direction of exposure 

 on the surface. As actually measured, it varies between 1 in 7 and 1 in 9, but 

 there are exceptions on either side, the least being 1 in 18 and the greatest 1 in 5. 

 Those with more rapid increase have been called 0. ventricosum, but no distinction 

 or satisfactory line can be drawn, and they are not of more interest or abundance 

 than those with a less rapid increase than the average. The body-chamber was 

 probably very long, though septa are rarely seen ; the longest actually proved is 

 three times its basal diameter. The aperture was prolonged towards the back and 

 front into tongues, of which one is longer than the other, and between these is a deep 

 lobe on the sides. The surface of the shell is rarely preserved, and thus it would 

 appear unornamented, but in several examples there are oblique transverse strise or 

 sharpish lines of growth most conspicuous on the body-chamber. The shell was 

 probably very thin, as it is so constantly broken down. The septa are oblique, 

 only rarely appearing direct, the obliquity being slight and in the same direction as 

 the lines of growth. They are distant £ to f the diameter, and their convexity is 

 small. On one example there is a longitudinal depression in each chamber, deepen- 

 ing behind at each septum, and representing the inner lobe. The siphuncle is 

 minute and central. The greatest diameter seen in flattened examples is 10 lines, 

 and the longest shell is 9 inches incomplete. 



Orthoceras prblevum, var. antiquior. 



The above description is taken entirely from Upper Silurian forms, but there 

 are examples in the Lower Silurian which can at most be indicated as a variety in 

 the absence of better material. These show the same differences in the rate of increase 

 depending on the compression, those actually observed showing a rate between 1 in 

 7 and 1 in 14. The aperture has not been seen of the same shape as in the 

 typical form, but as a sloping convex curve on the side : this may be due to imma- 

 turity. No transverse lines have been observed, but at the smaller end some 

 lines of growth and indications of longitudinal foldings, not due to compression. 

 In septal characters there is no difference. 



Relations. — Some of the fragments referred by Portlock to 0. regulare appear 

 to belong to the Lower Silurian variety of this. The characters are still too 

 obscure for certain comparison with any of Barrande's better known forms. The 

 two nearest British forms are 0. tenuicinctum, whose riblets are regular ornaments, 



