COLOCERAS BISTRIALE. 116 



CoLOCEEAS BISTRIALE, /. PhUUps, sp. Plate XXX, figs. 2 a — c. 



1836. Nautilus bistrialis, J. Phillips. Geology of Torksbire, pt. 2, p. 232, 



pi. xvii, fig. 21. 

 18M. — (Temnocheilus) bistrialis, F. M'Coy. Synop. Carb. Fobs 



Ireland, p. 20. 

 1891. C(ELONAUXiLUS BiSTUiALis, A. H. Foord. Cat. Foss. Ceph. British 



Museum, pt. 2, p. 130. 

 1893. CoLOCERAS BISTRIALE, A. Hyatt. Carboniferous Ceplialopods. Second 



paper. Greological Survey of Texas, Fourth 

 Annual Report, 1892, p. 451. 



Description. — Shell small, globose, composed of about two and a half whorls, 

 all exposed in a deep umbilicus. The apex is not seen, but it would appear that 

 the form of the young shell, so far as can be made out, does not materially differ 

 from that of the adult. The section of the whorls is roughly hexagonal, the 

 periphery broadly rounded with a shallow median depression ; from this the shell 

 slopes on each side to the edge of the umbilicus. The latter has a sharp keel, 

 which does not reach beyond the beginning of the body-chamber, whence the shell 

 is obtusely rounded up to the aperture. This has a deep hypouomic sinus (very 

 distinctly outlined on the body-chamber, PL XXX, fig. 2 c), with its edge sharply 

 reflected (fig. 2 a). Two fine but distinct ridges encircle the inner whorls just 

 within the umbilical keel, this ornamentation having suggested the specific name. 

 The test is quite smooth. Distinct traces of the Runzelschicht are seen upon the 

 cast of the body-chamber in one of the specimens before me.^ 



Neither septa nor siphuncle are known. 



Dimensions. 



Diameter of shell 



,, umbilicus (from edge to edge) 



,, ,, (from suture to suture) 



Height of outer whorl (without the test) 

 Breadth ,, ,, ,, . 



Affinities. — Nautilus glohatus, de Kouinck" (not J. de C. Sowerby), resembles 

 the present species in having a series of ridges encircling the umbilical walls, but 



^ A nearly perfect specimen (figured loc. cit.) contained in the Museum of Science and Art, 

 Dublin (Greological Survey Collection). 



2 Faune Calc. Carb. Belgique (Ann. Mus. Roy. d'Hist. Nat. Belgique, ser. Paleont., torn, ii), 

 pt, i, p. 95, pi. \, figs. 4ia,b (excl. figs. 2, 3). 



Specimen 

 PL XXX 



figured 

 , fig. 2. 



50 mm. 



33 



}} 



22 



jj 



18 



J' 



27 



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