﻿BRITISH FOSSIL CEPHALOPODA. 177 



Cyrtoceras compressum, Sowerby, PI. XVIII. figs. 1, 2. 



1838. Phragmoceras compressum, Sowerby in Murchison's 'Silurian System,' pi. 11, fig. 2, p. 621. 

 1873. Cyrtoceras? compressum, Salter, ' Cambrian and Silurian Fossils,' p. 160. 



Not 1843. Phragmoceras compressum, Portlock, ' Geol. Eeport,' p. 282. 



Type. — Only a portion is seen, in which the rate of increase apparently decreases 

 very rapidly so as to be almost zero at last. The curvature at first has a radius of 

 about \\ inches, but at last of 2£ inches only, the mean diameter of the shell 

 being 1 inch. The surface shows that the shell was thin, and ornamented by feeble 

 riblets of growth, 3 per line, which curve backwards and cut the septa obliquely. 

 The septa are about y 1 -^ the diameter of the whorl apart ; the sutures are not very 

 concave and bend back slightly towards the outside, so as to be sigmoid. The body- 

 chamber extends for \\ inches, at a less curvature than the rest, and shows no signs 

 of any contraction near the aperture. Curved length, 4^ inches. From the Lower 

 Ludlow rocks, Aymestry. In the Museum of the Geological Society. 



General Description. —The section was probably elliptic, but in none is it 

 perfectly preserved. The rate of increase is about 1 in 5 in the earlier portions, 

 or even greater from contortion, but reduces to almost zero at last. The mean 

 curvature is considerable, having a radius of 1^ inches when the mean diameter of 

 the shell is more than 1 inch. The body-chamber is as long as its basal diameter, 

 and shows no change at the aperture. The ornaments, when preserved, are 

 transverse sharp lines of growth, rather sigmoid in outline and cutting the sutures 

 towards the outside. The septa are approximate, concave on the side, but really 

 sigmoid ; they are very close, occasionally extremely so (fig. 2). The siphuncle is 

 external and bulbous. The type is as large as any. 



Relations. — The ornaments are something like those of Trochoceras cinereum, but 

 not so pronounced, and the present species appears to be a genuine Cyrtoceras. It is 

 also very like Trochoceras striatum when found in fragments, but complete shells are 

 immediately distinguishable. 



Distribution. — In the Lower Llandovery, Thrave (1) ; in the Coniston Grit, 

 Brownthwaite (1) ; in the Lower Ludlow of Aymestry (1), Leintwardine (1), 

 Aston (3), Stoke Wood (1), Ledbury (4), and of Usk (2) ; and from the Upper 

 Ludlow, Ludlow (2). 



It is also recorded by Lapworth from the Upper Silurian of the Pentlands ; by 

 Salter from Lower Ludlow, Parkes Hall; in the Catalogue of Western Scottish 

 Fossils, from Penkill. 



2 A 



