74 ORDOVICTAN AND SILURIAN BELLEROPHONTACEA. 



Localities. — Balclatchie; Ardmillan ; Dow Hill, Gtirvan. 



Remarks. — The non-expanded mouth, the long narrow dorsal slit succeeded by 

 the slit-band, the small angular umbilicus, the complete occupation of the narrow 

 dorsum by the slit-band and the general shape of the shell seem to necessitate its 

 reference to the genus Zonidiscus The revolving lines are unusual. There is also 

 some resemblance to some species of Gonradella and especially of Temnodiscus. 

 In many of the specimens the edges of the dorsal slit are squeezed together, 

 so that the slit is obliterated and the dorsum seems to be acutely carinated as in 

 Oxydiscus, but the narrow seam between the edges can generally be detected. 



All the examples of this species are in Mrs. Gray's Collection. 



4. Zonidiscus, sp. 



Specific Characters. — Shell discoidal, coiled in flat spiral, composed of 4 — 5 

 whorls, rather slowly increasing in size to mouth, very slightly overlapping, cordate 

 in section, higher than wide, with acutely angular dorsum, convex sides, and short 

 steep undefined umbilical slope. Umbilicus large, shallow, open, exposing inner 

 whorls ; no definite umbilical edge. [Slit-band present, very narrow ?] Orna- 

 mentation unknown. 



Dimensions.- — Height of shell, 23*0 mm.; height of outer whorl near mouth, 

 8*5 mm. 



Horizon. — Lower Llandovery. 



Locality. — Canilo Hill, Builth ; ? Blaen-y-cwm. 



Remarks . — There are three specimens [28057 — 59] of this shell in the Jermyn 

 Street Museum, but none are sufficiently well preserved to allow of a full specific 

 diagnosis. The slit-band cannot be seen in the casts or impressions, and the 

 ornamentation is not discernible. The shell labelled B. discus, Edgell MS. [28045], 

 from the Lower Llandovery of Blaen-y-cwm, may be identical, but is itself too 

 fragmentary for a satisfactory determination. The outer whorl does not increase 

 in size so rapidly as in Z. grayi or Z. grayi var. shallochensis, and more resembles 

 the species described by Lindstrom as Cyrtolites discus, Lindstr., 1 which is the 

 type of the genus Zonidiscus. 



Genus SALPINGOSTOMA, Roemer. 



Generic Characters. — Shell symmetrically coiled in one plane; whorls numerous, 

 enlarging gradually, scarcely embracing ; umbilicus large, open. Mouth abruptly 

 expanded at maturity, forming a trumpet-like aperture, subcircular in shape ; 



1 Lindstrom, op. cit., p. 84, pi. vii, figs. 18 — 21. 



