42 ORDOVICIAN AND SILURIAN BBLLEROPHONTACEA. 



than one-third the diameter of the shell. Outer whorl large, gradually increasing 

 in size and more rapidly in Avidth than height towards month, becoming twice 

 as broad as high ; umbilical edges acute or subangular, becoming rounded or 

 obtuse near mouth and in old shells ; umbilical slopes steep, high ; dorsum strongly 

 arched, convex, becoming lower and flattened towards mouth ; centre of dorsum 

 raised into a low, broad, rounded, gently elevated revolving ridge carrying slit-band ; 

 a narrow, sharp, revolving lateral keel is situated on each side at about three-fifths 

 the distance between the slit-band and the umbilical edge, becoming obtuse or nearly 

 obsolete near mouth ; interspaces between central ridge, keels and umbilical edges 

 more or less concave. Slit-band well-marked, gently concave, with sharply 

 raised edges and crossed by strong equidistant regular lunuloe. Mouth transverse, 

 very slightly expanded at sides, with short broad median slit and gently rounded 

 apertural lobes. Surface of shell crossed by regular closely-placed, strong, arched 

 transverse minutely granulated thread-like lines, bending back to meet slit-band 

 at about 30°. 



Dimensions. — Height of shell, 13'0 mm; width of mouth, 15*0 mm. 



Horizon. — Lower Ordovician : Balclatchie Group. 



LocaJities. — Balclatchie and Ardmillan, Girvan. 



Bentarls. — The affinities of this species with Tetranota sexcminata, Ulr. & 

 Scof.,^ are close. The ornamentation in our species is rather finer, and the dorsum 

 is more arched, except towards the mouth. Apparently this is the form which has 

 been frequently entered as '' Belle rophoii trilobatns, Sowerby," in lists of Girvan 

 fossils. The type specimens are in Mrs. Gray's Collection. 



2. Tetranota carrickensis, var. craigensis, nov. Plate VIII, figs. 12, 13. 



Varietal G/iaracters. — The only difference between the Craighead and Ardmillan 

 specimens of T. carrickensis seems to lie in the position of the lateral keels, which 

 in the Craighead form are placed rather nearer the slit-band, being about half-way 

 between it and the umbilical edges. But none of the Craighead specimens in 

 Mrs. Gray's Collection are well preserved or perfect. It is, however, possible that 

 this form is a distinct species. 



Horizon. — Lower Ordovician : Stinchar Limestone Group. 



Localltj/. — Craighead, near Girvan. 



3. Tetranota carrickensis, var. etheridgei, nov. Plate VIII, figs. 14, 15. 



Varietal Characters. — Slit-band narrower than in T. carrichensis, and surface of 

 shell with the transverse lines crossed by finer ones, causing a minute cancellation 

 of the surface. 



1 XJlrich and Scofiold, op. cH., p. 878, pi. Ixv, figs. 3-9. 



