﻿144 BRITISH FOSSIL CEPHALOPODA. 



convex septa, and a circular section with central siphuncle, which features are 

 inconsistent with the characters of the present species ; such differences, as well as 

 its closer septa, separate it from Sowerby's true species. The closeness of its septa 

 and general characters place it very near to 0. excentricum, from which its greater 

 rate of increase appears to differentiate it. Some Bohemian species, as 0. insons, 

 are similar to this ; but they appear to have a slower rate of increase. It seems 

 very probable that some of the Lower Silurian forms referred to 0. gregarium may 

 belong here, but it cannot be said to be proved. 



Distribution. — In the Bala Series of Rhiwlas, Bala (8), and at Sandley (1), and 

 probably also in the Lower Silurian of Waterford. 



Orthoceras pictum, Blake, PI. XIII. fig. 5. 



Type. — The section is circular, and the rate of increase is 1 in 6. Details of 

 body-chamber are not shown. The shell is perfectly smooth on the surface, but 

 shows longitudinal bands of colour, alternately white and brown, some wider and 

 some narrower, about 12 per diameter. The septa appear to be direct, and to have 

 a convexity of \ the diameter. The siphuncle is central and moderate in size. The 

 diameter is 1 inch, and the length nearly 3 inches. From the Bala Series of Bala. 

 In the Museum of Practical Geology. 



General Description. — The other examples being fragmentary, no confirmation of 

 the form is possible, but in the body-chamber the rate of increase is perhaps less. 

 The aperture is bounded by an undulating curve without any constriction below. 

 The body-chamber is probably longer than its basal diameter, as no septa are seen 

 in the specimen showing the aperture. The colour bands are variable in their 

 distance and breadth, the dark colour being as broad as the light. The greatest 

 diameter seen is If inches. 



Relations. — The presence of coloured bands may be held to differentiate this 

 species from others. Nevertheless, a comparison must be made with such as have 

 not yet shown their surface, as, for example, 0. audax, which agrees in the rate of 

 increase and the slight convexity of the septa ; but it has a less circular section, and 

 the septa undulate, while the siphuncle is slightly excentric. These differences, 

 though small, relieve us from uniting the species without proof of coloured bands 

 on 0. audax. The central siphuncle distinguishes our species from 0. pellucidwn, 

 Barrande, which is similarly coloured. 



Distribution. — In the Bala Series of the Bala district (4) and of Acton Scott (1). 

 The specimens are in the Museum of Practical Geology, and of Owens College, 

 Manchester. 



