46 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



Specific Characters. — Transversely triangular, tumid ; anterior end rounded, 

 excavated above by lunule ; and its anterior-superior angle almost a right angle, 

 and much below the level of the umbones. Posterior end is gradually compressed 

 and bluntly pointed and narrowed from above downwards by a rapid descent of 

 the superior border. 



Umbones contiguous, situated at the junction of the anterior and middle 

 thirds of the shell ; swollen and prominent, forming the apex of the triangular 

 shell. The posterior part of the hinge-line slopes rapidly from the umbones. 

 External ligament is very prominent, and terminates abruptly ; posterior to it is a 

 V-shaped trench, becoming shallower and narrower posteriorly between the 

 bevelled edges of the hinge-plates. Inferior border is bluntly rounded in front, 

 straight or sinuate behind ; in the latter case the posterior-inferior angle of the 

 shell has a downward direction. 



There is often a broad constriction posterior to the greatest tumidity of the 

 shell, directed downwards and backwards. 



Hinge-plate consists of three parts. Anteriorly there is an elongated triangular 

 surface, bevelled slightly at the expense of its lower border, the upper border 

 becoming slightly thickened into an elongated ridge, which forms the lower 

 border of the lunule; this ridge terminates suddenly at a point just anterior to or 

 beneath the umbo, and is excavated posteriorly by a broad concavity, deeper 

 anteriorly, but becoming gradually shallower as it passes into the posterior part of 

 the hinge-plate. This concavity extends from just beneath the point of the umbo 

 to the inferior margin of the hinge-plate, which is lost in the depression. The 

 concavity is transversely striate ; the striae appear to be continuous with those of 

 the lunule, which dip down into it. Posterior to the concavity there is a broad 

 plain vertical surface, elongated triangularly, bevelled at the expense of its upper 

 border, which, narrowing as it passes backwards, becomes lost in the external 

 surface of the shell at its superior posterior angle. 



Interior normal. Anterior-adductor scar almost marginal, deep, and oval. 

 Posterior-adductor scar shallow. Surface with fairly strong concentric lines of 

 growth, which are contiguous anteriorly, becoming separated as they pass over 

 the body of the shell, often with a slight sinuation posteriorly, when they become 

 reflected at a rounded right angle, to be lost in the superior border along the 

 groove of the external ligament. Shell thick. 



PI. I, fig. 1. Another specimen, PI. II, fig. 1. 

 Length antero-posteriorly. 90 mm. 55 mm. 



Dorso-ventrally . . 58 mm. 35 mm. 



From side to side . . 42 mm. 20 mm. 



Localities. — In the Upper Coal-measures of Scotland. Rankinstone Ironstone, 



