﻿118 BRITISH FOSSIL CEPHALOPODA. 



is f \ across the longer diameter, and is small in size on the septal surface. The 

 greatest length seen is 3§ inches, and the greatest diameter f- inch. 



Relations. — The peculiar feature of this species is the presence of excessively fine, 

 and yet perfectly preserved, acute riblets. It is curious that both the Bohemian and 

 our own fossils are pseudomorphs in calcite, and it may be from this cause that such 

 riblets are preserved. Barrande separates from this species, under the name of 

 O. capillosum, some larger shells which have longer body-chambers, a triflingly less 

 rate of increase, and the riblets only about 20 per line. This latter is very widely 

 distributed in time, ranging from stage B to H ; and if 0. argus were to be con- 

 sidered a local variety of it, with which the British species more nearly agreed, it 

 would be more satisfactory, as ours also has a wide range of time. It differs from 

 the other British species with fine riblets in their excessive number, and from most 

 of them in their directness. 



Distribution. — The four examples referred to this species, and which I am unable 

 to separate by any positive character, come from widely different horizons. One is 

 from the Middle Bala of Holbeck Grill, and this does not show its siphuncle well, as 

 there is an appearance, which I think delusive, as if it were lateral. The second 

 is from the Wenlock Shale, Kingswood Fordon ; the third is from the Lower Ludlow 

 of Ludlow ; and a fourth in the Upper Silurian, bed H, of the Pentland Hills, may 

 be the same. 



Orthoceras expansum, Blake, PI. VI. fig. 15. 



Query 1846. Orthoceras lineare, M'Coy, ' Sil. Foss. of Ireland,' p. 9. (Not of Minister.) 



Type. — The shell is nearly flattened; the rate of increase is greater than 1 in 6. 

 The ornaments are fine undulating riblets, not very elevated ; these are not parallel 

 to the septa, and rise more on one side than on the other, being somewhat oblique ; 

 at a diameter of 7 lines they are 12 per line. The septa are nearly direct and very 

 remote, viz. f- the diameter ; the siphuncle is not seen. The length is 2 inches, and 

 the greatest diameter 7 lines. From the Bala Series of Desertcreat. In the Museum 

 of Practical Greology. 



General Description. — Another example referred to this shows the section to be 

 elliptic in the ratio of 13 to 11 ; the rate of increase is the same as in the type. The 

 ornaments, though sharp, have a tendency to imbricate upwards. In the other 

 examples they are fewer than in the type, namely, 5 to 10 per line, but some varia- 

 tion must be admitted in these ornamented species. The septa are nearly direct, as 

 in the type, and so are not parallel to the ornaments ; they are distant in another 

 example ^ the diameter, and the sutures are a little undulating. No siphuncle has 

 been seen. The greatest length seen is 3 inches, and greatest diameter 10 lines. 

 One example shows the mending of the shell during life. 



