96 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



Genus Planktoceras, Hyatt, 1893. 

 Planetoceras globatum, /. de G. Soiverby, sp, Plate XIX, figs. 5 a, h, 6. 



1821. Nautilus globatus, J. de C. Sowerby. Min. Conch., vol. v, p. 129^ 



pi. cccclxxxi. 

 1844. — (Temnocheilus) glouatus, F. M'Coij. Synop. Carb. Fosa. 



Ireland, p. 21. 

 1860. Temkocueilus globatus, E. Griffith. Journ. Geol. Soc. Dublin, vol. ix, 



p. 77. 

 1891. C(ELONAUTiLUS GLOBATUS, A. H. Foovd. Cat. Eoss. Cepb. British Museum 



pt. 2, p. 127, fig. 20. 

 1893. Planetoceras globatum, A. Hyatt. Carboniferous Cephalopods. Second 



paper. Geological Survey of Texas, Fourth 

 Annual Eeport, 1892, p. 421. 



Defscription. — Shell subglobose, consisting of about two and a half volutions, 

 the inner ones exposed in a A^eiy deep umbilicus with almost vertical sides and a 

 central vacuity of moderate size. The shell increases rapidly. The apex is 

 conical. The periphery is flattened in the young shell, and there is a distinct 

 angle at the edge of the umbilical vacuity which persists to the completion of 

 about one and a half volutions, where it develops into a strong keel which is 

 slightly inflected, so that its edge faces the umbilicus. The periphery, flattened 

 in the young shell, becomes rounded in the adolescent and adult stages, but it 

 tends towards the aperture to be again somewhat depressed. The last whorl, by its 

 lateral expansion and the depression of the peripheral area in approaching the 

 aperture, assumes a laterally spreading form, with a very wide, dorso-ventrally 

 contracted aperture. 



Near the aperture the body-chamber frees itself from the penultimate whorl 

 and arches upwards tangeutially to the original coil. 



The aperture has a somewhat broad hyponomic sinus, on each side of which two 

 broadly rounded lobes or "crests" project. At the angles just below the keel 

 there is a slight notch or wave in one of the specimens before me, but the line of 

 the aperture connecting these points has not been seen. 



The body-chamber occupies half of the last whorl. 



The septa are rather approximate, their distance in a young shell being about 

 one fifth of the dorso-ventral diameter. 



The siphuncle is nearly central. 



The surface of the test is smooth, only extremely fine lines of growth being seen 

 when it is well preserved. In a young and almost complete specimen, in which the 



I 



