SANGUINOLITES OMALIANUS. 373 



Specific Characters. — Shell inequilateral, transversely ovate, gibbose, twisted 

 on itself, obscurely and obliquely carinate. The anterior end is short, and 

 narrowed from above downwards by the descent of its upper border, which is 

 occupied by a very long and sloping luuule. The anterior border is elliptically 

 curved. The inferior border is slightly sinuous about its centre, and becomes 

 markedly convex behind, where it is curved bluntly upwards to pass into the 

 posterior margin, which is straight and obliquely truncate from above downwards 

 and backwards above, but bluntly rounded and somewhat produced below. The 

 hinge-line is shorter than the greatest antero-posterior diameter, much depressed 

 in front, and produced, straight or slightly arched upwards behind. The 

 umbones are tumid, incurved, and pointed, somewhat obliquely set, much raised 

 above the hinge-line, and situated in the anterior fourth of the valve. Passing 

 downwards and backwards to the postero-iuferior angle is a well-marked 

 oblique ridge, with a slight curvature having its concavity upwards, angular 

 above but more obtuse below. The ridge becomes somewhat less marked as it 

 approaches the edge of the valve ; it separates the dorsal slope from the rest of 

 the valve. Passing from the umbo towards the postero-superior angle is an erect 

 curved ridge, which separates the dorsal slope from the large, broad, and deep 

 escutcheon. The valves are very convex from above downwards, but irregularly 

 curved from before backwards. The anterior portion of the valve is moderately 

 convex, the greatest convexity being oblique and along a line from the umbo to a 

 point a little in front of the postero-inferior angle. Immediately in front of this 

 convexity is a well-marked sinus, which becomes broader as it approaches the 

 inferior margin, when it often appears as a sinuosity in its curvature ; in some 

 specimens this sinus is almost obsolete. The dorsal slope is large, broad, and 

 hollow ; and the escutcheon is long, comparatively broad, and excavated. 



Interior. — The arrangement of the muscle-scars seems to be identical with 

 that which obtains in 8. variahilis. The pallial line is entire. The hinge has 

 not yet been observed. The concavity of the interior is marked with concentric 

 grooves and ribs. 



Exterior. — The surface-marking is very variable, but consists of concentric 

 grooves and ribs. In some specimens the ribs bifurcate after passing across the 

 siaus, and are close and narrow. In others they are few and thick, but they dis- 

 appear altogether on the lower portion of the valve ; and on the dorsal slope the 

 grooves and ribs terminate at the oblique ridge. Under the microscope many of 

 the ribs and sulci are finely striated concentrically. The posterior slope is also 

 covered with fine striae, arranged parallel to the posterior margin. Shell very 

 thin. 



Dimensions. — Fig. 22, PI. XL, a specimen from Tomdeely, co. Limerick, in 

 the collection of the Geological Survey of Ireland, measures — 



