﻿BRITISH FOSSIL CEPHALOPODA. 131 



Section Imbricati. 

 Orthoceras Avelinii, Salter, PI. VI. figs. 1, 2. 



1854. Orthoceras Avelinii, Salter in Murcbison's ' Siluria,' Foss. fig. 4, p. 48. 



1866. „ „ Salter, 'Memoirs of the Geol. Surv.' vol. iii. pi. 11 b, fig. 18, 



p. 356. 

 Syn. 1875. Orthoceras caereesiense, Hicks, ' Quart. Journ. Geol. Soo.' vol. xxxi. pi. 11, figs. 8-10, 



p. 189. 



Type. — The examples in the Museum of Practical Geology, which give the data 

 for Salter's description, are, no doubt, his types ; but none of them agrees exactly 

 in dimensions with his figure. The two specimens, which may form part of a single 

 shell, give the following characters. The section is elliptic, the diameters being the 

 ratio of 8 to 7. The rate of increase is 1 in 7 in both fragments. The ornaments 

 on the smaller consist of undulating rounded elevations, rising on the side of the 

 siphuncle, but no finer lines are preserved. On the other, these elevations are not 

 proved, though the shell is more prominent at the septa, which might be due to the 

 resistance to compression. The finer ornaments are upward imbrications, about 

 30 in the diameter of the shell, or from 4 to 8 per line, but rather irregular; 

 they are a little oblique. The septa are parallel to them, but somewhat undulating, 

 -j^j- the diameter apart, corresponding, perhaps, to the elevations. The siphuncle 

 is \ across the diameter on the side to which the septa rise. The united length of 

 the specimens is 4| inches ; greatest diameter, f- inch. From the Lower Llandeilo 

 of Cefn Gwynlle, Shelve. In the Museum of Practical Geology. 



General Description. — The section is not seen in any other examples. The rate 

 of increase is seldom seen so great as in the type, owing, perhaps, to the speci- 

 mens being mere surface-marks ; and this is especially the case with those called 

 0. caereesiense. The ornaments, in the form of low rounded elevations, are 

 constantly present in the young shell and not always absent in the adult. The 

 finer ornaments are well seen on the external cast figured by Dr. Hicks (loc. cit., 

 fig. 10). At the smaller end they are nearly straight, and look from the inside 

 more like broken edges of septa (which of course they are not) than ornaments : 

 they are here 3 per line, which is not less than -^ the diameter preserved, and that 

 is doubtless only a fraction of the whole ; towards the larger end they are closer, and 

 are -^ to -33- the apparent diameter. Other examples from Shelve have similar 

 curious imbrications, but usually these are worn off. The septa are not, but the 

 position of the siphuncle is confirmed. The type is the largest seen. 



Relations. — Dr. Hicks states that his 0. caereesiense differs from 0. Avelinii in 

 being broader and having closer striae. The breadth is merely due to their flattening 



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