﻿128 BRITISH FOSSIL CEPHALOPODA. 



siphuncle is central. The greatest length seen is 2^ inches ; the greatest diameter, 

 5 lines. 



Relations. — When no distinctive characters are derivable from the septa and 

 siphuncle, the only marks by which to separate these longitudinally ribbed species are 

 the number of the riblets and the rate of increase. The latter, from compression, 

 is seldom a safe guide, and to judge of the former we must at least have a semi- 

 circumference preserved. Fragments are therefore difficult to localise, but in the 

 Bala rocks there are certainly some whose riblets are better marked and more 

 remote than in others : to these the name of 0. lineatum may be reserved. These are 

 comparatively delicate shells, admitting of no real comparison with such giants as 

 O.filosum, although that may be similarly ornamented. No specimens have been 

 seen which justify the introduction of such names as 0. laqueatum or 0. striato- 

 punctatum, which are founded on details of ornament not to be demonstrated 

 on British fossils. 



Distribution. — Fragments which may be referred to this species occur in the 

 Middle Bala Beds of Holbeck (1) ; in the Upper Bala of Coldwell (1) ; in the Bala 

 Beds of Desertcreat (6), and in the Lower Silurian of county Tipperary (3) ; also 

 the Coniston Flags at Hawkshead (1) and Coniston (1). 



Fossils under this name, or its synonyms, have been described by M'Coy as 

 common in the grey calcareous slates at G-lengraff, Gralway ; but those I have seen 

 from here show no satisfactory characters. Mr. Baily records it also from Lower 

 Silurian Beds at Ballycar, Ballenbrook, and Killoskehan, co. Tipperary. It ranges 

 also, according to authors, through Sweden, Norway, and Russia, so that, though 

 absent from the Welsh area, as far as at present known, some such form was a 

 widely distributed one during the Bala period. 



Orthoceras lineatum, var. tenuistriatum, Portlock, PI. VII. figs. 7, 13. 



1843. Orthoceras tenuistriatum, Portlock, ' Geol. Eeport,' pi. 28, fig. 1, p. 370. (Not 



of Minister.) 

 1852. „ „ M'Coy, ' British Palaeozoic Fossils,' p. 317. 



1854. „ „ Morris, 'Catalogue of Brit. Fossils,' p. 312. 



1870. „ lineatum, Barrande, ' Syst. Silur. de Boheme,' pi. 438, p. 704. 



General Description and Relations. — A few specimens associated with the above 

 species, with others having a larger range, appear to differ from it only in the 

 greater number of the riblets. To these the name given by Minister to a Devonian 

 fossil has been usually applied. This, however, has been described by him as having 

 a thick shell, these being thin, and it is scarcely likely that our specimens have a 

 closer relationship to those of so much later date than to their associates in the same 

 beds ; as a specific name, tenuistriatum is, therefore, inapplicable, but may be used 



