72 ORDOVICTAN AND SILURIAN BELLEROPHONTACEA. 



becoming slightly concave near keel, and swelling out towards base so as to form 

 a narrowly rounded or subangular umbilical edge; umbilical slope short, steep or 

 vertical. Umbilicus open, rather deep, about one-fourth the diameter of shell in 

 width, and situated at rather less than half its height. Outer whorl increasing 

 rather rapidly in height and embracing greater part of shell, with long, narrow 

 median open slit in lip extending back from peristome for about one-third of the 

 whorl ; remainder of outer whorl provided with narrow slit-band raised on carina 

 having sharp edges, and crossed by thick strong subequidistant lunulas sometimes 

 thickened and rising above carina so as to give it a crenulated appearance. Mouth 

 higher than wide, with reflexed inner lip. Surface of shell ornamented with 

 rather unequal transverse fine lines directed back rather obliquely from umbilical 

 vd^c and running nearly straight to base of carina, where they bend back suddenly 

 to meet slit-band at about 45°. 



Dimensions. — Height of shell, 27"0 mm.; height of outer whorl near mouth, 

 140 mm. ; width of base of mouth, ll - mm. 



Horizons. — (1) Drummuck Group (Starfish Bed), Upper Ordovician ; (2) 

 ? Redhill Beds. 



Localities. — (1) Thraive Glen, Grirvan ; (2) PPrendergast Place, Haverfordwest. 



Remarks. — The edges of the open slit in the anterior part of the outer whorl 

 are usually crushed together, but its posterior end is generally well marked by 

 the sudden appearance of the carina. The slit-band is well preserved in specimens 

 which retain the shell. Probably this is the species from Drummuck described by 

 Salter 1 as "like Bellerojphon acutus, but with wide umbilicus." 



The reference of this species to Spit/.'s genus Zonidiscus cannot be doubted, 

 for it combines the shape and general appearance of < ).riiili,<cn.< with the slit-band 

 and slit of Cymbnlaria, ami it closely resembles the type of Zonidiscus {Cyrtolites 

 discus, liindstr.), though that species comes from a higher strat igraphical horizon. 



Several specimens of this species in the Hunterian Museum are labelled 

 Bellerophov (Trematoiiotus) ca,rinatus, with a note stating that the specimens show 

 "the row of close perforations on the keel." This appearance of perforations 

 seems due to the obliquity and prominence of the equidistant thick lunula?, which 

 rise up in a series of little projections along the keel so as to separate slightly 

 depressed and concave elongated areas on the slit-band, thus giving a spurious 

 resemblance to oval foramina. A side view of the shell makes the carina appear 

 crenulated, but after examining many specimens in various states of preservation 

 with and without the shell, it seems certain that there are no true perforations. 



The best specimens of this species are in the Hunterian Museum, Glasgow, and 

 come from the Drummuck Group. Several examples also occur in Mrs. Gray's 

 Collection. 



The Redhill specimens which are in the Sedgwick Museum are too poor for 

 satisfactory identification. 



1 Salter, 'Quart. Jouni. Geol. Soc.,' vol. vii (1851), p. 172. 



