TRIBOLOCERAS FORMOSUM. 75 



Portlock begins his description by comparing his species with a Goniatite : 



" A larger and flatter shell than Professor Phillips's,^ which is described as 

 * very depresssed ' back (in adults) truncate ; umbilicus open ; fine transverse 

 bent striae." Professor Phillips's species was about half the size of this, 

 and the back is represented as quite flat ; in this it is slightly concave and 

 narrower. The sides are much flattened ; and the umbilicus, represented by 

 Phillips as open, but very small, is evidently in this also very small, if not entirely 

 closed. 



" The surface is not sufficiently preserved to exhibit striae ; but the septa can be 

 traced, and appear more suitable to the Nautiloid than Goniatitic type. Diameter 

 of disc 3*5 inches, thickness at back '3 of an inch; sides much flattened; outer 

 edge shallow concave; septa very concave. 



" This diff'ers from Nautilus sulcatus, and some other species with flattened or 

 concave backs, by the volutions being concealed. 



'^ Locality.— Derrjloran, Tyrone, in light pinkish limestone." 



Family Thiboloceratid^. 

 Genus Trtboloceeas, Hyatt, 1883 (emend. 1893). 

 Tbiboloceeas FORMOSUM, sp. nov. Plate XXIT, figs. 1, 2. . 



P1844. Temnocheilus CEENATrs, F. M'Coy. Synop. Carb. Eoss. Ireland, p. 21, 



pi. ii, fig. 9 (fragment). 



jDescription. — Shell discoid, composed of two volutions having a very deep 

 funnel-shaped umbilicus with a large central vacu^t3^ The w^horls, hexagonal 

 in section in the young, become digonous in the adolescent and adult stages. The 

 body-chamber occupies apparently half a volution. It is slightly detached from 

 the preceding whorl near the aperture. 



The periphery in the young and adolescent stages is broad, and is divided into 

 three distinct zones, the centre being occupied by a low median elevation, bounded 

 on each side by shallow furrows ; ridged elevations, which are a little higher than 

 the median one, fill the spaces on each side of the furrows and slope downwards 

 to the keeled umbilical margin. From the latter there is an abrupt slope to the 

 suture of the shell. 



1 " Go7iintites truncafus" Phill., ' Geology of Yorkshire,' pt. 2, 1836, p. 234, pi. xix, figs. 20, 21 

 (= Olyphioceras truncatum, Phill., sp., ' Cat. Foss. Ceph. British Museum,' pt. 3, 1897, p. 175, 

 fig. 82). 



