﻿BRITISH FOSSIL CEPHALOPODA. 87 



ribs are more direct, and the longitudinal ornaments are more regular and constant. 

 In 0. calamiteum the ribs are very rounded and the longitudinal lines very fine ; and 

 the same may be said of 0. bilineatum of Hall, which might, however, be possibly 

 included among the varieties of this. In 0. pseudo-calamiteum the longitudinal lines 

 are more delicate, sharp, and continuous, and the ribs are less oblique. On the whole, 

 it is unfortunate that the more typical varieties should have received names that are 

 not applicable ; and if it should be proved that the type chosen does not really belong 

 to the series, a new name would be required for the latter. 



Distribution. — The great majority of the specimens come from the Bala Shales of 

 Desertcreat (16) ; but one is also figured from beds of the same age at Ardwell, and 

 the specimen referred by M'Coy to 0. bilineatum comes from Grirvan. 



Orthoceras velatum, Blake, PL III. figs. 12, 12a. 



1865. Orthoceras velatum, Salter, MS. Catalogue of Fossils in the Museum of Practical Geology. 



Type. — The section is nearly circular, inclining to quadrate ; the rate of increase 

 is 1 in 18. No characters of the body-chamber are seen. The larger ornaments are 

 gently undulating ribs, somewhat oblique and distant -|- the diameter. These are 

 covered by fine longitudinal and transverse riblets, each 18 per line, dividing the 

 surface into square cancellations. The septa have a convexity of \ the diameter, 

 and appear to cut the ribs on the upward slope. The siphuncle is moderate and 

 central. The length is 2 inches, and the diameter more than ^ inch. From Bala 

 Limestone, Cynwyd. In my collection. 



General Description. — The section, though generally compressed, is nearly circular 

 in two or three instances, or at least equiaxial. The rate of increase in smaller 

 examples is about 1 in 9, but decreases at the larger diameters. Some variation may 

 be supposed in the ornamentation, due to distortion, preservation, and growth. The 

 ribs can never be called separate ; but even in those associated in the same beds with 

 the type, they are, at times, almost acute, at others much less marked, and have a 

 tendency, when highest, to imbricate downwards. They are probably always oblique, 

 the maximum obliquity being 12°. Their distance in the smaller examples is ^, but 

 in the larger ^ the diameter. The finer ornaments certainly change character some- 

 what in the length of the shell, the longitudinal lines being widest apart at first and 

 ultimately getting much closer ; some among them may become dominant, or, on the 

 other hand, they may become so small as scarcely to be seen. Thus either one set or 

 the other of these lines may prevail without the total absence of the other, or there 

 being other characters that would justify a separation. In a curious specimen from 

 Haverfordwest, which may belong here, the longitudinal lines are twice as numerous 

 as the transverse, and quite subordinate to them. The septa, as seen in an example 



