﻿BEITISH FOSSIL CEPHALOPODA. 183 



shell. No siphuncle is seen. Length of specimen, 22 lines ; greatest diameter, 

 2 1 lines. From the Bala Beds, Desertcreat. In the Museum of Practical G-eology . 



General Description. — The specimen figured by Portlock as 0. breviconicum has 

 been much distorted and compressed, the result being a greater apparent expansion. 

 The shape of the present section shows this, being altogether irregular. Allowing 

 for this compression, all the other features agree with the above, and confirm the 

 lines of growth nearly parallel to the sutures, the slight undulation of the sutures, 

 and the flatness of the septa. It shows also a minute and central siphuncle. Another 

 smaller specimen shows the septa as usual, at about -^ the diameter apart, and the 

 lines of growth curving backwards on the convex side. 



This brevicone Cyrtoceras is of especial interest, as being of that group which 

 will lead on to the Phragmocerata, the flatness of the septa being a point of alliance. 

 There is not, however, in these the slightest sign of contraction near the aperture, 

 nor of change in the general shape of the body-chamber. 



Relations. — There cannot be the slightest doubt, after an examination of the 

 specimens, of the identity of the two species united. This species differs from 

 C. sonax in its less curvature and more rapid increase, and in the position of the 

 siphuncle. 



Distribution. — In the Bala Beds of Desertcreat (4). I have not seen any that 

 could be certainly referred to this species from other localities, but Lapworth records 

 it from " Middle Silurian," Grirvan, and an example in the Museum of Practical 

 Geology from Rhiwlas Bala has a similar rate of increase and like ornaments, but 

 its section is nearly circular. This may perhaps be the true uncompressed form of 

 the species, but this single specimen has not sufficient characters to prove it. 



Cyrtoceras extricatum, Blake, PI. V. figs. 10, 11, 11a. 



Syn. 1838. Lituites articulatus (part), Sowerby in Murchison's ' Silurian Syst.' pi. 11, fig. 7 



(not the others). 

 1852. „ „ M'Coy, ' Pal. Foss.' p. 323. 



Type. — The section is circular. The rate of increase in the septal portion is 

 unseen, the example chosen as type consisting of body-chamber only, which decreases 

 towards the aperture (fig. 10, reversed by the artist). The mean radius of curvature 

 is 2|- inches at a diameter of f inch. The body-chamber is four times as long as its 

 basal diameter. The aperture is circular and not marked by any constriction. The 

 ornaments are strong radial non-separate ribs, about ^ the diameter apart ; the shell 

 surface is not preserved. The septa are parallel to the ribs, and the convexity is 

 nearly equal their interval. The siphuncle is small and nearly central. Length, 

 2-^ inches. From the Lower Ludlow, Mocktree. In the Ludlow Museum. 



General Description. — Another example of body-chamber, figured (fig. 11), shows 



