THE LAMELLIBRA.NCHS OF THE SILURIAN ROCKS OF GIRVAN. 513 



umbones are small, pointed, and incurved, slightly raised, placed a little anterior to the 

 middle line. The valves are obliquely swollen, and a very compressed and hollowed 

 dorsal slope is separated from the rest of the shell by a bluntly rounded oblique ridge 

 which passes from the umbo to the posterior inferior angle, becoming less angular as it 

 approaches the border of the shell. 



Interior. — The hinge consists of three short radiating teeth separated by grooves 

 posteriorly, and in front of four very fine oblique teeth. The anterior adductor muscle 

 scar is small and placed low down, immediately within the margin of the valve ; the 

 posterior is shallow and placed in the hollow of the dorsal slope, distant from the 

 hinge line. 



Exterior. — The surface is ornamented with fine, somewhat irregular concentric lines 

 and striae of growth. 



Dimensions. — Fig. 18, PL IV., a left valve, measures : antero-posteriorly, 37 mm. ; 

 dorso-ventrally, 26 mm. ; elevation of valve, 8 mm. 



Locality. — Lower Llandovery of Mulloch Hill. 



Observations. — The only example in the collection consists of a cast of the interior 

 and its mould, showing the external surface of the valve. The hinge plate is well 

 exposed. 



It will be noted that Salter says his shell was rather compressed. I have described 

 this example as moderately gibbose, and we differ as to the position of the umbones, for 

 I do not think that his figure warrants the description : " The beak is anterior, beyond 

 the anterior fourth." Salter's type appears to have a more angular ridge from the 

 umbones to the posterior inferior angle than the Scotch specimen, but I fancy there has 

 been some compression of the former, and that this character is largely accidental. 



Cyrtodonta transversa, sp. nov. (PL IV., figs. 19, 20.) 



Specific Characters. — Shell below medium size, transverse, gibbose. Anterior end 

 much narrower than the posterior in the dorso-ventral diameter, very inequilateral. 

 The anterior end is narrowed, its border rounded. The inferior border is elongate and 

 descends, becoming almost straight posteriorly. The posterior border oblique, bluntly 

 rounded below, almost straight above. The hinge line is straight, raised posteriorly. 

 The umbones are elongate, obtuse, inrolled and twisted forward, placed far forwards 

 but not terminal. Proceeding from the umbo towards the postero-inferior angle is a 

 blunt, rounded gibbosity, above which the dorsal slope is so much compressed 

 as to be concave. Anteriorly the valve is slightly compressed above the middle 

 lower border. 



Interior. — The hinge plate is normal. Adductor muscle scars normal. 



Exterior. — Ornamented by concentric grooves and rugose lines of growth. 



Dimensions. — Fig. 19, PL IV., a left valve, measures: antero-posteriorly, 19 mm. ; 

 dorso-ventrally, 13 mm. 



