﻿BRITISH FOSSIL CEPHALOPODA. 83 



Orthoceras durinum, Blake, PL III. figs. 3, 3a. 



Syn. 1859. Orthoceras undulosostriatum, Salter in Murchison's ' Northern Highlands, &c.,' 

 Quart. Journ. Geol. Soc, vol. xv. pi. xiii. figs. 25, 26, p. 375. (Not of Hall.) 

 Query, Orthoceras arcuoliratum, Salter, loc. cit., p. 375. (Not of Hall.) 



Type. — This is the specimen figured by Salter (fig. 26) as above. Its section is 

 now oval in the ratio of 5 to 6, but may naturally be circular. The rate of increase 

 is 1 in 45. No body-chamber is seen. The surface is entirely worn away and 

 beekised ; the apparent ribs are due entirely to unequal weathering, and such orna- 

 ments, to judge from the specimen, may or may not have been present. The septa 

 are moderately convex, and are oblique 22° on the sides, but direct dorsally and 

 ventrally : in one of these positions they are less worn away over^ a limited area, and 

 this has produced an appearance of a sinus which does not really exist. The siphuncle 

 is not actually seen, but it is probable that this preservation may be due to its pre- 

 sence, in which case it is marginal, on the side to which the septa slope back. There 

 are depressed lines on the casts of the chambers parallel to the septa ; these may be 

 due either to internal projections, or may indicate a line of absence of the encrusting 

 deposit on the septa. Length \\, in diameter \ inch. Prom the Durness Limestone. 

 In the Museum of Practical Geology. 



General Description. — The section has not been observed circular, and the 

 tapering is very slow. If, as I think, the specimen referred to by Salter as 

 0. arcuoliratum, belongs here, and it agrees in other characters, then there 

 were certainly rather acute ribs of 20° obliquity, distant f the diameter, and this is 

 probable in any case. The septa are oblique 20°-24°, and correspond to the 

 interval between the ribs ; they are distant -f to -^ the diameter. In none has 

 the siphuncle been seen, but in one, besides the type, are some peculiar features, 

 which indicate it as marginal on the side to which the septa slope back, as from the 

 great obliquity of the latter we might expect ; in this case there is no unequal 

 wearing, and hence no appearance of a sinus, but there are two impressed lines 

 which lie between and are parallel to each pair of septa over a limited area, which I 

 interpret as the limit, on each side of the neck of the septum, of the usual encrusting 

 deposit. The peculiar appearance indicated by Salter in his fig. 25 is due merely 

 to some septa having been broken through and their remaining edges being worn 

 away. The type is the largest seen. 



Relations. — This species has undoubtedly some considerable resemblance to 

 Hall's 0. unduloso striatum, but in that the siphuncle is nearly central, whereas, 

 if my interpretation of appearances is the right one, 0. durinum has it lateral ; 

 also, the arch in the septa and in the supposed ribs being only due to weathering, 

 one great point of resemblance disappears. No finer ornaments are here for 



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