CARBONICOLA ACUTA. 51 
Awturacosta Faprtctt, Achepol. Niederrheinisch.-Westfilisch. Steinhohlengebirge, 
Atlas, 1880-83, Suppl., pl. iv, figs. 61, 62. 
— Roémert, Achepol. Ibid., p. 186, pl. xli, figs. 1—20. 
— acuta, Ward. Trans. N.-Stafford. Inst. Min. and Mech. Engineers, 
vol. x, p. 128, pl. i, fig. 8, 1890. 
Specific Characters.—Shell elongate, transversely ovate, tumid; anterior end 
short, bluntly rounded, swollen. Posterior end produced, gradually compressed and 
narrowed by the approximation of the superior and inferior borders, with a more 
or less acutely pointed posterior-inferior angle, above which the border is obliquely 
truncate. Hinge-line straight, about two-thirds the length of the shell. Umbones 
gibbous, broad, fairly prominent, not contiguous ; beaks deflected very slightly 
forwards, situated at the junction of the anterior and middle thirds of the shell. 
Lunule elongated, and, when both valves are together, ovate; striated above by the 
lines of growth curving round and becoming reflected into the inner face of the 
umbo; smooth and plain for a narrow space along the edge of the hinge. 
Neutral margin bluntly rounded anteriorly, the curve being continuous with the 
anterior border; at about the centre of shell it becomes straight, very slightly 
sinuous, and directed upwards. On each side of the posterior part of the upper 
border, which is somewhat erect, the shell becomes slightly constricted longitu- 
dinally. Cartilage external, extending about half the length of the hinge-line. 
Shell thick, almost massive at umbones. Surface with coarse irregular con- 
centric lines of growth. 
Anterior- and posterior-adductor scars deep ; the former almost marginal, con- 
taining at times pearl-like growths. 
The hinge apparatus is very variable; but three distinct types may be recog- 
nised, which I purpose describing as a, 8, and y. These different forms occur in 
the same bed, and are connected by transitional forms. 
Form a (PI. V, figs. 1 and 3—16).—The hinge-plate may be divided into three por- 
tions. In the right valve anteriorly there is an elongated triangular surface, which 
may or may not be bevelled and facetted at the expense of the lower border. The 
superior border is slightly thickened or ridged to form the cardinal tooth, behind 
which is a broad concavity, becoming shallow posteriorly, when it passes into a 
broad vertical smooth plate, which rapidly narrows, to be lost on the inner sur- 
face of the shell near the posterior end. In the left valve anteriorly the lunule 
passes into an elongate concavity, while there is a more or less marked lip below 
the lunule and anterior part of the depression, smooth or facetted, and at times 
bevelled at expense of upper border. Posterior to this concavity, which terminates 
abruptly, is a thickened boss, the cardinal tooth, which may or may not be 
separated from the posterior part of the hinge-plate by a depression, which 
receives an accessory tooth, if one be present in the right valve. The posterior 
