DISCITOCKRAS LEVEILLEANUM. 103 



1893. DisciTOCERAS Leveilleanum, A. Hyatt. ('iirboniterous Cc|.lialopod8. 



Second paper. Geological Survey of Texas, 

 Fourtb Aunual Report, 1892, p. 435. 



Description. — Shell discoid, somewhat compressed, rather slowly tapering, 

 composed of about three and a half whorls having rounded angles, and all exposed 

 in a wide and shallow umbilicus with a central vacuity of moderate size. The 

 section of the whorls is of a modified hexagonal form, the angles of the whorls in 

 the adult stage showing a tendency to become rounded, so that the distinction 

 between the umbilical, lateral, and peripheral areas almost disappears. The apex 

 is bluntly conical, and is covered to its extreme tip by the ornamentation of the 

 test. In the young shell the whorls are more generally rounded than at a later 

 stage — the adolescent — when they become angular at the umbilical margin, this 

 angularity gradually becoming softened until it finally disappears in the region of 

 the aperture. The latter has an open sigmoid curvature, with a moderately deep 

 hyponomic sinus upon the periphery, where also the lines of growth become 

 coarse and irregular. 



The length of the body-chamber extends to about half a volution. The shell 

 becomes detached from the penultimate volution in approaching the aperture 

 (PI. XXVII, figs. 1 a, 2 a). 



The ornaments of the test ai'e strikingly beautiful. They consist of a series 

 of fine, acute, longitudinal ridges, which, very close-set in the young shelU 

 gradually widen out as the whorls increase in diameter, ceasing abruptly at the 

 end of about two and a half volutions. These ridges cover the sides of the shell, 

 the peripheral area, and the umbilical declivity, extending sometimes along the 

 latter iu three or four widely separated ridges, considerably beyond the place 

 where the lateral ones have become obsolete. 



Fine, very close-set, and remarkably regular transverse lines of growth cover 

 the whole surface of the test, giving rise to crenulations where they cross the 

 longitudinal ridges. These lines of growth, becoming coarser in the more mature 

 shell, are seen to be of the nature of narrow bands; when magnified, they vary 

 greatly in width, sometimes two, sometimes even four, occupying the space of a 

 millimetre. Their general texture, however (if the expression may be used), 

 suggests uniformity to the eye when unaided by the lens. They are admirably 

 shown in the larger figure of this species (PI. XXVII, fig. 2 (/). 



Dimensions. Laiyc spcciiuen from Claiie, 



Musi'tiiii of Science 

 siiid Alt, Dublin. 



Diameter of shell . . . .124 mm. 



Height of outer whorl . . . 45 ,, 



Diameter of umbilicus (appro.ximate) . . 70 ,, 



