66 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



1878. Nautilus mctauilis, L. G. de KonincJe. Faune Calc. Carb. Belgique 

 (Ann. ]Mus. Roy. d'Hist. Nat. Belgique, Paleont., 

 torn, ii), pt. 1, p. 121, pi. XXV, figs. 2 a — c. 



1891. DisciTES co-sirnv.ssvs, A. H. Fooi-d. Cat. Foss. Ceph. British Museum, 

 pt. 2, ]). 91, fig. 11 (iiot EllipsoUtes compressus, 

 J. .Sowerby, Min. Conch., 1813, vol. i, p. Si, 

 pi. sxxviii). 



1S93. .ApiiELKCKiiAS MUTAiiil.E, A. Hyatt. Carboniferous Cephalopoda. Second 



paper. Geological Survey of Texas, Fourth 

 Annual Report, 1892, p. 41.3. 



Description. — Shell of a very distiuctly discoid shape, the sides tapering rapidly 

 towards the periphery ; much compressed ; composed of three and a half to four 

 quickly increasing volutions, the sides of which are completely exposed in a wide 

 and shallow umbilicus, with a rather large central perforation. The section of the 

 whoi'ls in the adult shell is hexagonal, or, taken as a whole, sagittate with truncated 

 apex; in the young the umbilical angles are inconspicuous, and the dorsal or anti- 

 peripheral area is rather more rounded. The initial point is conical, and is 

 sometimes detached from the second whorl (de Koninck). As the shell grows the 

 umbilical walls become gradually deeper and somewhat concave, the suture of 

 the shell having sometimes a slight rim in the last whorl. The edge of the 

 umbilicus forms a thick, rounded angle owing to a thickening of the shell sub- 

 stance. The sides are slightly inflated in the adolescent stage but become flat in 

 the adult; they terminate at the edge of the periphery in a sharp and prominent 

 keel ; the periphery is deeply concave and very narrow. The impressed zone is 

 marked by a median ridge bordered by tw'o grooves corresponding with the 

 ventral furrow and its bordering keels. The body-chamber occupies a little more 

 than half the last whorl; it becomes separated from the penultimate whorl in adult 

 individuals for a distance of 18 mm. to 38 mm. 



The septa are very numerous and deeply concave ; thirty-six can be counted to 

 one whorl in a shell whose diameter is 93 mm. (PI. XXL fig. 3). Where the whorl 

 has a dorso-veutral diameter of 22 mm. they are 6 mm. apart, at a diameter of 

 15 mm. their distance is reduced to 5 mm., and again at a diameter of 7 mm. they 

 are 3 mm. apart (PI. XXI, fig. 3). Their distance apart is thus very gradually 

 augmented. The sutures make rather a deep backward curve on the sides of 

 the shell and on the periphery. 



The siphuncle is a little above the centre, and maintains that po.sition through- 

 out the whole growth of the shell. 



Ornamentation is only developed in the young shell ; it consists of fine and 

 regular longitudinal ridges covering the sides, and, to a certain distance, and more 

 sparsely, the dorsal area. About eight of these ridges occur where the Avhorl, 

 bordering the central vacuity, has a diameter of 5 mm. ; they are strongest and 



