CARBONICOLA ELEGANS. 81 



17. Carbonicola elegans (Kirldnj), 1880. Plate XX, figs. 12 — 15, 15 a. 



Plefropiiorus elegans, Kirkby. Quart. Journ. Geol. Soc, vol. xxxvi, 18S0, 



p. 586, " On the Zones of Marine Fossils in 

 the Calciferous Sandstone Series of Fife." 



Specific Characters. — Shell transversely produced, cylindrically curved, riot 

 exceeding 25 mm. in length. The anterior end is small and much narrower dorso- 

 ventrally than the rest of the shell. Its border is rounded in front, but hollowed 

 out above by the lunule ; it passes imperceptibly into the lower border, which is 

 almost straight in direction, but posteriorly it rises very gently towards the 

 posterior inferior angle. Its posterior border is rounded, the upper slope being 

 more rapid than the inferior, and the postero-inferior angle is the most posterior 

 part of the shell. The hinge-line is formed of two portions which meet at an 

 obtuse angle posterior to the beak of the umbo. The upper border of the shell is 

 swollen. 



The umbones are broad and obtuse, rising gradually from the surface of the 

 shell without any ridge anterior or posterior, but in front thejr are excavated by 

 the lunule ; they are situated about the junction of the anterior and second fourths 

 of the length of the shell. The beaks are small, but curved over and pointed 

 downwards and forwards, and terminate close to the hinge-line. The lunule is 

 comparatively broad, and well marked. The whole shell is regularly swollen, 

 more so above, and the valves have a rapid curvature from above downwards. 

 The dorsal slope is somewhat flattened, and is always above the articulating hinge- 

 line ; the inner edge of the superior border is bevelled at the expense of its upper 

 portion, forming with the opposite valve a V- s h a P e d trench in the centre of which 

 the valves articulate, and on either side of this articulating line is a longitudinal 

 groove for the external ligament. 



Interior. — The anterior adductor scar is almost marginal, deep and circular, 

 the deepest part of the scar being at the upper and posterior part. The accessory 

 adductor scar is placed above and internal to the anterior adductor; a strong 

 umbonal rib bounds the anterior adductor behind. The posterior adductor scar 

 is situated far back, just within the postero-superior angle of the shell; it is 

 shallow and inconspicuous. The pallial line is simple, dotted, remote from the 

 margin of the shell. Between the hinge-line and the superior border is a 

 longitudinal groove in casts to receive the edge of the shell. Very obscure 

 radiating lines are seen on the surface of casts. The hinge is a simple form of the 

 type which obtains in Carbonicola aquilina. There are no anterior or posterior 

 lateral teeth, but the edges of the hinge-line are bevelled below the umbo, the 

 left valve being bevelled at the expense of its lower edge, to form a hinge. 



11 



