THRIiNCOCEKAS HIBERNICUM. 101 



Thrincocekas Hibbbnicum, a. H. Foord, sp. Plate XXVI, figs. 2 a, b. 



1891. GrYROCEEAS HiBEENicTTM, A. U. Foord. Cat. Foss. Ceph. British Museum, 



pt. 2. p. 64, fig. 5. 



1893. RiNECEEAS HiBEENicuM, ^. l?//a«. CarbouiferDua Ceplialopods. Second 



paper. Geological Survey of Texas, Fourth 

 Annual Report, 1892, p. 425. 



Description. — Shell discoid, composed of two and a half to three whorls, with a 

 wide, rather shallow umbilicus, having apparently a large central vacuity. The 

 section is subquadrangular and hexagonal ; the periphery broad, compressed, very 

 slightly convex ; the sides flat, the umbilical declivities not very steep ; their junction 

 with the sides not well defined, but in the adolescent stage an obtuse angle is here 

 formed, which becomes rounded off towards the aperture. 



The whorls increase rather slowly in diameter; the last one near the aperture 

 detaches itself for a very short distance from the penultimate whorl. The 

 peripheral area only is concealed by the embracing whorls, the sides of the inner 

 whorls being entirely exposed. 



The body-chamber appears to extend to about half a volution ; the outline of 

 the aperture has not been seen. 



The septa are numerous ; there are about twenty-two in a complele volution 

 in the young shell. They curve abruptly backwards upon the sides and periphery, 

 forming an acute angle upon the edge of the latter. The last two septa are 

 5 mm. apart. 



The siphuncle has not been seen. 



The ornamentation consists of numerous acute, longitudinal ridges, of which 

 there are nine or ten comparatively coarse ones upon the sides and umbilical 

 declivities, and eleven finer ones upon the peripheral area. The ridges are finer 

 and less prominent at the lower part of the umbilical declivity, near the suture 

 of the shell, and become gradually stronger and more distinctly defined, as well as 

 wider apart, until the edge of the periphery is reached, upon the surface of which 

 they are very delicate, though preserving their distinctness. 



The finer ornaments consist of a series of very fine transverse lines, which, 

 thickening as they cross the ridges, give to the latter a strikingly crenulated 

 character. 



These lines cross the whorls with a slight backward deviation, but on the 

 peripheral area they describe a very deep, backwardly-directed sinus, which 

 indicates a hyponomic sinus at the aperture of similar form. 



