84 ORDOVICIAN AND SILURIAN BELLEROPHONTACEA. 



similar to those on dorsum, curving outwards slightly on each side of a median 

 impressed groove running up to marginal dorsal sinus. 



Dimensions. — Height of shell (without oral expansion), c. 20 mm. ; height of 

 mouth, c. 47 mm.; width of mouth, c. 44 mm. 



Horizon. — Upper Bala Beds. 



Locality. — Desertcreat, Tyrone. 



Remarks. — There is only one specimen, the type [27996] of this species, 

 and it is in the Jerniyn Street Museum ; the apertural side which Portlock 

 correctly figured is complete, but the umbilicus and dorsal parts are obscured by 

 matrix. The raised continuous median band and the ornamentation mark it off 

 at once from the typical Tr. dilatatus (Sow.), though Portlock says that Sowerby's 

 description "exactly suits " it. The foramina on the slit-band are very indistinct, 

 but seem to be small and contiguous. The raised band and general aspect some- 

 what resemble Tr. longitudinalis, Lindstr., 1 but the finer and more regular ornament 

 ;md the greater sharpness of the umbilical edges are distinctive features. The 

 band, as in that species, seems to become narrower and sharper and to lose its 

 foramina on the oral expansion. 



Genus PHRAGMOSTOMA, Hall (emend. Koken). 



Generic Characters. — Shell of same general shape and structure as Trematonotus, 

 but with dorsum possessing only one opening or a small number of orifices. These 

 orifices only occur on the outer whorl towards the mouth and are borne on a 

 median ridge bordered on each side by a groove; the rest of the dorsum of the 

 outer whorl is smooth. 



The generic name is here employed in the manner followed by Perner (op. fit., 

 p. 121), who adopted Koken's 2 definition. The single species described below 

 is doubtfully referred to this genus. 



1. Phragmostoma decipiens, sp. now Plate XIII, figs. 5 — 7. 



1852. Bellerophon dilatatus, Sowerby, McCoy, Syn. Brit, Pal. Foss. Woodw. Mus., fasc. ii, p. 309 

 (non Sowerby). 



Specific Characters. — Shell subglobose, subdiscoidal, of 4 — 5 broad transverse 

 whorls, slightly overlapping to about one-fourth or one-fifth their height ; umbilicus 

 conical, deep, rather large, being rather more than one-third the diameter of the 

 shell, exposing all the inner whorls; umbilical edge subangular to rounded; 



1 Lindstrom, op. cit., p. 86, pi. iv, figs. 1 — 7. 



2 Koken, ' Die Leitfossilien ' (1896), p. 99. 



