30 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



Whidborne, ' Devonian Fauna of the Soutli of England,' Palajontograpliical 

 Societ}', 1890, vol. for 1881'), and similar forms ornamented with frill-like lamellae 

 in North America (cf. James Hall, ' Paljeont. New York,' 1879, vol. v, part 2). 

 In all these, certain characters itcalling the Silurian forms are to be traced; 

 these ai'e the numerous se])ta and marginal siphuncle, sometimes exogastric, 

 sometimes endogastric, the highly (jrnate shell being perhaps the only distin- 

 gui.shing mark that can be applied to them as a group. 



The Carboniferous species, so far as they are known, present, on the whole, a 

 more simple type of structure than that of their Silui'ian ancestors as represented 

 in the rich series of forms found in the Bohemian basin. The shells are 

 generally more rapidl}' tapering and less strongly and uniformly curved, and the 

 septa much less numerous in the Carboniferous species, which thus represent a 

 generalised type in which the features that distinguished the ancestral forms have 

 become greatly modified. 



The tendency in this expiring race to a more simplified structure is still more 

 strongly exemplified in the species to which I have given the new name Eusthe- 

 noceras, a passage form, in all that relates to the structure of the adult shell (the 

 embryo is not known), from Ortlioceras to Cj/rtoreras, using these words in a 

 somewhat wide sense. 



I may here state that I do not count among species of Gijrtuceras all the forms 

 attributed to it by de Kouinck (' Calc. Carb. Belg.,' 1880) ; on the contrary, I 

 would exclude all but the following : — Gyrtoceraa (Meloceras) cornu, de Kou. ; 

 G. (M.) (tens, de Kon.; G. (M.) Verneidliannm , de Kon. ; G. (M.) arachnoideum, 

 de Kou.; G. (ilf.) Gesneri, Mart.; G. (M.) nnjasum, Flem.; G. (M.) rostrahim, 

 de Kon.; G. (ilf.) digitus, de Kon.; G. [M.) i iiiperltum, de Kon.; G. {M.) acus, 

 de Kon.; G. (ilf.) Nysti, de Kon.; G. (M.) repertam, de Kon. 



The fragment named by de Koninck Gijrtoctiraa cornu-bovis is difficult to 

 allocate, though it seems on the whole to be more akin to Gyrtoceras than to any 

 other group. Gyrtoceras Antilope, de Kon., another fragment, has only one 

 Cyrtoceran character, viz. a slight curvature, quite insufficient to establish its 

 connection with the genus to which it is referred by de Koninck. It has 

 considerable resemblance to a species described in the first part of this memoir 

 (1897, p. 19) under the name of Ort/ioceras liiherHicum, which is also slightly 

 curved. The latter has a more rapidly increasing diameter and somewhat wider 

 septa, and the elements of the siphuncle are not so inflated nor so wide and 

 flattened as they are in the Belgian species. The two species may, nevertheless, 

 fairly be compared, and it was by an oversight that this was not done under the 

 description of the Irish fossil. 



LocoUty. — St. Doulagh's, county of Dublin. 



