﻿BEITISH FOSSIL CEPHALOPODA. 81 



outline of the aperture, if really preserved, is slightly undulating. The specimen is 

 a cast, and hence no ornaments appear. The septa slope in a slightly different 

 direction to that of the aperture, and are about \ the diameter apart. The length is 

 If inches, and the greatest breadth 10 lines. From the Bala Series of Haverford- 

 west. In the Museum of Practical Geology. 



Distribution, tyc. — No other specimen has been seen, but the remarkably large 

 angle of increase can scarcely admit of any doubt of the distinctness of this species. 

 In shape it is not unlike 0. Barrandei, but has not its ornaments. It is the earliest 

 of our Brevicones. 



Group II. Longicones. 



Section Annulati. 



Orthoceras mendax, Salter, PI. III. fig. 1. 



1859. Orthoceras mendax, Salter in Murchison's 'Northern Highlands, &c. ;' Quart. Journ. 



Geo!. Soc, vol. xv. pi. xiii. fig. 24, p. 374. 

 1872. Orthoceras sp., Salter in Murchison's 'Siluria,' Foss. gr. 27, fig. 6, p. 165. 



Type. — Salter's figure is made up of three pieces not belonging to the same 

 individual, though of the same species. He states the section to be circular. The rate 

 of increase is 1 in 8. No portion of the body-chamber is seen. The ornaments are 

 transverse, rather acute, ribs, almost direct, and distant 3- the diameter. No finer 

 ornaments are preserved. Salter states that the young is nearly smooth, but this 

 may be from wear. The septa correspond to the elevation of the ribs, and are 

 therefore distant ^ the diameter, and nearly direct ; their convexity is slight. The 

 siphuncle is said to be eccentric, but this appears to me due to compression, as only 

 half of the upper part is preserved ; moreover it is drawn (fig. 246) as central. 

 There are enlargements of the septa in the neighbourhood of the siphuncle, which 

 I take to be the remains of the obstructing deposits ; and thus the exterior sheath of 

 the siphuncle, which usually represents it, is gone, and we have left the usually 

 corneous inner tube, passing through the septa without any signs of constriction. 

 This inner tube is thus perfectly smooth, except for a longitudinal fold, as if it were 

 a membrane in too small a case, and has a diameter -§ that of the shell ; the whole 

 siphuncle being about |- the diameter. The largest single fragment is 2 in. long by 

 10 lines. From the Durness Limestone. In the Museum of Practical Geology. 



General Description. — I am not certain I have seen the specimen supposed to be 

 young, without ribs ; but two examples, otherwise valuable, have so suffered from 

 ' beekisation,' as many of the fossils from this limestone have, that every sign of ribs is 

 lost. Including these, and others with extra sharp ribs, as examples, the section was 



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