﻿BRITISH FOSSIL CEPHALOPODA. 129 



as a varietal one until other associated characters justify a separation. In the type 

 the number of riblets in a space equal to the diameter would be 14. In those 

 included in the present variety, the number ranges from 24 to 48 in the same 

 distance. The examples from Upper Silurian rocks appear to be of larger size than 

 the others, and one (fig. 13) approaches 0. originate in appearance, except for the 

 more numerous riblets, and is associated with it. 



Distribution. — In the Bala Beds of Desertcreat (? — Portlock's type not seen), and 

 of Tipperary (1) and Mullock (2) ; in the Lower Llandovery of Haverfordwest (1) 

 and the Upper Llandovery of Marshbrook ; in the Wenlock Shale of Builth (1). 



It is also recorded by Harkness and Nicholson from Randy Pike, and a similar 

 form occurs at Kirkcudbright. 



Okthoceras bullatum, Sowerby, PI. XII. figs. 4, 5. 



1838. Orthoceeas bullatum, Sowerby in Murchison's 'Silurian Syst.' pi. 5, fig. 29, p. 612. 



1852. „ „ M'Coy, 'Pal. Foss.' p. 313. 



1867. „ „ Dixon, 'Woolhope Nat. F. Club, Fossil Sketches,' No. 1, fig. 1, 



p. 136. 

 Syn. 1838. Orthoceras striatum, Sowerby, in explanation of figures as above. (Not 0. stria- 

 tum, M.C., t. 58.) 



1847. „ substriatum, D'Orbigny, ' Prodrome,' p. 54. 



Type. — The specimen is flattened so that the section is unknown, and the rate of 

 increase becomes 1 in 9. Only the commencement of the body-chamber is seen. The 

 ornaments are regular longitudinal impressed striae with flat interspaces, distant six 

 per line at a diameter of 19 lines. The sutures are now oblique 3°, and distant \ the 

 compressed diameter. The siphuncle throughout the space in which it is seen has 

 2 lines' diameter, though the shell has increased its size. The form of its elements 

 changes with growth ; in the earliest part they are bulbous between the septa, and 

 later they are flatter or cylindrical, thus accounting for the want of increase in the 

 size. The total length is 8^ inches, and the greatest diameter is 19 lines. From 

 the Upper Ludlow of Ludlow. In the Museum of the Geological Society. 



General Description. — In uncompressed specimens the section is circular. There 

 is in some a slight curvature of radius 15 in. The rate of increase at first is 1 in 6, 

 but in the body-chamber it decreases to almost zero. The length of the body- 

 chamber is not more than twice its basal diameter, and has a very slight contraction 

 below the aperture, the outline of which is undulating. The ornaments vary in 

 character according to the preservation, showing three states. If the shell is well 

 preserved, as in uncompressed examples (fig. 4), they consist of very numerous 

 sharp longitudinal riblets, standing up on a flat surface ; on average specimens these 

 are from 7 to 10 per line, but near the apex may be as many as 36 per line. If on 

 the contrary the shell is as usual an internal cast showing the septa, the ornaments 



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