TRIEOLOCERAS FORMOSUM. 77 



the latter excepting a small portion of it near the aperture, where it is quite 

 smooth. 



The whole of the surface of the shell is covered with fine transverse lines of 

 growth, which, in crossing the ridges, are thickened into small but conspicuous 

 nodes which ai'e somewhat elongated transvei'sely. This crenulation gives a 

 characteristic appearance to the ornamentation, which henco resembles that of 

 Vestinautihis seviigJaber (PL XXII, figs. 3, 4«, h). 



Dimensions. 



Diameter of shell 



,, umbilicus (from edge to edge) 



,, ,, (from suture to suture) 



Height of outer whorl (dors o- ventral) about . 

 Width of periphery of outer whorl . 



Affinities. — It is possible that the fragment of a body-chamber from Tirlecken, 

 Shrule, county of Longford, described by M'Coy under the name of Temnocheilus 

 crenatus, may belong to the present species, but its imperfect condition will leave 

 this always a matter of uncertainty. M'Coy's specimen is in the " Griffith Collec- 

 tion " of the Science and Art Museum, Dublin. 



Allied to Triboloceras formosum is T. Meyerianmn, de Kouinck, sp. ; the dis- 

 tinctions between them are, however, quite clear. The section of the whorls 

 (adult) in the latter is distinctly scutiform, the height in proportion to the width 

 being much greater than in T. formosum ; thus in the latter, where the height 

 (dorso-veutral) of the whorl is 31 mm., the width, or lateral diameter, is 50 mm. 

 In T. Meijerianum, where the height of the whorl is 44 mm., the width is 48 mm., 

 a very slight difference compared with that which obtains in T. formosum.. 

 Another element of divergence between the two species is to be found in the 

 ornaments, the ridges on the periphery in T. Mei/erianum being differently 

 disposed from those of T.fonnosum; those of the former arc greatly elevated in 

 the median line, four of the ridges standing out with great prominence and giving 

 a pointed aspect to this part of the shell. The ridges upon the umbilical 

 declivities are also more numerous in 2\ Meijerianum than in T. formosum. 



Remarks. — This handsome shell, though here assigned to Triboloceras, seems 

 to be a passage form between that genus and Vestinautilus ; its very deep, 

 crateriform umbilicus and broad peripherj' connecting it with the latter, from 

 which, however, it is distinguished by the digonous foi'm of its whorls and the 

 numerous and prominent longitudinal ridges ornamenting its surface. It has, 

 moreover, been remarked by Prof. Hyatt that Triboloceras Meijerianum is a 



