70 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
a lower level than the rest of the shell. The inferior border, convex anteriorly, 
soon becomes almost straight. The posterior part of the shell is produced, 
uniformly tumid, and is hardly compressed till near the borders. ‘The posterior 
border may be rounded or obliquely truncate from above downwards, or almost 
pointed. The upper borders of the valves are flattened and expanded, so that the 
greatest convexity of the shell is on a level with the hinge, this rendering the 
posterior slope very blunt and angular. 
The umbones are anterior, slightly everted, obtuse, with the apices directed 
forwards and remote. They rise in front by a well-marked curved fold which has 
a well-defined edge, and takes its origin from the surface of the valve below the 
upper edge of the valve, which appears to be compressed anteriorly to and above 
the fold. Posteriorly the umbones pass into the posterior slope very gradually. 
The hinge-line is below the level of the upper surface of the valves, and each valve 
is channelled above by a longitudinal groove which lies between the hinge: edge, 
which is compressed and raised upwards, and the posterior umbonal slope. These 
grooves are well marked in casts. The external ligament is inserted into the 
bottom of these grooves, where the fibres appear to be continuous with the lines 
of growth, as they turn over the upper border of the shell and dip into the 
groove. 
The surface is marked by fine or coarse lines and striz of growth, markedly 
oblique in their arrangement to the long axis of the shell and eccentric. These 
striae are crowded and very convexly curved at the anterior end, but they gradually 
become further apart and almost straight till they are rapidly reflected at an angle 
over the umbonal slope, when they become directed forwards and inwards. The 
surface is often subsulcated. 
The hinge-plate consists of two portions which are placed at an angle with each 
other, the direction of the anterior part being upwards and backwards, and that 
of the posterior part almost horizontal, but there is no plate of shell connecting 
the two portions. I quote King’s original description of the hinge apparatus :— 
“ Teeth, one in each valve below (and anterior to) the umbo, rather low and massive ; 
crown of tooth of right valve excavated anteriorly (and below) and ridged pos- 
teriorly ; crown of tooth of left valve ridged anteriorly and sloped posteriorly.” 
These teeth may be facetted. The posterior part of the hinge-plate joining the 
anterior part at an acute angle is flattened, or sometimes rounded, and excavated 
as in Anodon for the posterior terminal attachment of the external ligament, and 
ultimately is lost on the inner surface of the shell. 
Above the hinge-plate is a longitudinal groove formed by the curving round and 
terminating of the lines of growth, which serves for the lateral attachment of 
the ligament. 
Jnterior.—The anterior-adductor scar is almost marginal, the front part being 
