﻿BRITISH FOSSIL CEPHALOPODA. 217 



Relations. — The dimensions as well as the ornaments clearly distinguish this 

 from Cyrtoceras compressum, though possibly that species is its nearest ally among 

 the forms that had been described in Portlock's time. 



Distribution. — Found only in Bala Beds at Desertcreat (3). The matrix is a 

 peculiar light porous brown rock, which looks very much like volcanic ash, whence 

 the name has been suggested. 



Tkochoceras corntt-arietis, Sowerby, PI. XXI. figs. 6, 6a ; PI. XXVIII. fig. 5. 



1839. Lituites cornu-arietis, Sowerby in Murchison's ' Sil. Syst.' pi. 20, fig. 20, p. 643 



(both, varieties). 

 1843. „ „ Portlock, ' Geological Keport,' pi. 28b, fig. 7. 



1849. Lituites Sowerbianus, D'Orbigny, ' Prodrome,' p. 1. 

 1852. Lituites cornu-arietis, M'Coy, 'Palaeozoic Fossils,' p. 323. 

 1852. „ „ Salter, Appendix to ' Pal. Fossils,' p. viii. 



Type. — There are two so-called varieties of this species, according to Sowerby, 

 and they have apparently somewhat different characters. In the example, var. a, 

 which is merely an external cast, the rate of increase is 1.42, last whorl .33 of the 

 diameter. The several whorls are just in contact, all exposed, fitting closely to the 

 centre, and not quite symmetrical ; they are 3 in number, and there is no straight 

 portion. Section apparently rounded ; thickness t 7 q- the whorl-breadth. The ribs are 

 backward curving, not clearly separate, becoming wider apart with age, and covered 

 by parallel riblets. Diameter 16 lines. From Caradoc Sandstone, Corton, near Pres- 

 teign. In the other example, var. /3, the rate of increase is only 1.2, and the last 

 whorl .3 of the whole, so that the inner whorls are partially concealed. It is now 

 compressed, and thus has an acute front, but the sides are uniformly convex. The 

 ribs are sharp, and about 27 per whorl, curving backwards towards the convex side, 

 and are covered by parallel riblets. No septal characters are seen. From the Lower 

 Llandovery Shales. Both the specimens are in the Museum of the Geological Society. 



General Description. — In all the collections examined there are but few examples 

 of either of these forms, and they do not range themselves definitely in two groups 

 as though there were two species, but are most satisfactorily considered as belonging 

 to one rather variable one. An extremely instructive specimen (PI. XXI. fig. 6) 

 from the Bala Beds, Sholeshook, seems further to justify this union. Its rate of 

 increase on one side is 1.45, and the last whorl is .37 of the whole, thus agreeing 

 fairly well with the first variety, but it shows decided signs of asymmetry. There 

 are 2^ whorls, but all is septate. The ornaments are seen to change with growth. 

 In the first 1^ whorls they are merely fine riblets, only occasionally rising to a 

 stronger one, but later on fairly strong ribs are developed, still covered by the 

 riblets, and all curving back as in the type. We are thus permitted to corisider that 



2 F 



