940 MR. T. H. AVITHERS ON 



about one-third the extent of the valve from the apex ; above the 

 umbo the valve is usually produced abi'uptly into an acute angle ; 

 lateral margin longer thaii the basal margin, almost straight in 

 its lower part and usually sharply bent inwards and upwards 

 above a line with the umbo ; basal mai^gin indistinctly marked off 

 into three almost equal parts, the two outer parts being obliquely 

 inclined towards the apex ; lower occludent margin slightly 

 convex, about the length of the basal margin ; upper occludent 

 margin about half the length of the lower occludent margin, 

 sometimes nearly in line with it, but more often abruptly bent 

 from the umbo towards the lateral margin, and in some cases 

 almost in line with the umbo and neai-ly at right angles to the 

 lower occludent margin. Almost all the specimens have a strong 

 ridge extending from the umbo to a point just above it on the 

 lateral margin, and above this ridge the narrow portion of the 

 valve is sharply bent downwards and upwards, and forms a deep 

 trough which is bounded by the raised upper occludent margin ; 

 three further ridges, not to be seen in some specimens, extend 

 from the umbo to the basal margin, one to the basilateral angle, 

 a second to one-third of the distance from the rostral angle, and 

 a third to midway between the two. On the inner surface there 

 is a deep pit for the adductor scutorum ; the inner occludent 

 margin is much thickened, and above the mnbo a portion of the 

 valve is bent downwards at right angles to the outer surface in 

 some specimens, and in others it is sharply bent inwards ; when 

 the occludent margin is almost in line with the umbo, this 

 downwardly and inwardly bent portion of the valve almost 

 entirely overhangs the pit for the adductor scutorum, and forms 

 a roof to the ujjper j^art of the innei' surface. 



Tergum (PI. XCIV. fig. 7) subrhomboidal, slightly convex 

 transversely, with a feebly marked curved ridge extending from 

 the apex to the basal angle at about one-fifth the width of the 

 valve from the carinal margin, and from this lidge the valve 

 slopes steeply to the carinal margin ; uj)pei- half of valve curled 

 towards the scuta, and in most valves away from the opposing 

 tergum; apex acute; basal angle sometimes acute, and sometimes 

 slightly and obliquely truncated ; carinal margin convex when 

 continuous, but when formed of two almost equal portions, as is 

 usually the case, the upper margin is convex and the lower margin 

 almost straight ; scutal or lateral margin straight, almost equal 

 in length to the concave occludent margin, which has a slightly 

 raised border ; the shape of the upper carinal and occludent 

 margins is influenced by the extent to which the upper half of 

 the valve is curved towards the scuta, a character Avhich is 

 very variable. Along the occludent margin, especially in those 

 examples conspicuously bowed away fi'om the opposing tergum, 

 a, portion of the valve is bent downwards and shai'ply inwards to 

 form an obtusely angular ledge on the inside of the valve ; this 

 ledge, as well as that formed on the inner occludent margin of 

 the scutum, was no doubt for the protection of the animal's body, 



