PARALLELODON CANCELLATUS. 145 



Cf. Paeallelodon cohoides, de KonincTc, 1885. Ann. Musee Hist. Nat. Beige, 



vol. xi, p. 159, pi. xxiv, fig. 52. 



Specific Characters. — Shell of moderate size, transversely produced, convex. 

 The anterior end is small, compressed, and the narrowest part of the shell. Its 

 border, commencing above near the hinge-line, with which it makes a right angle, 

 rapidly descends backwards in a broad curve, so that the anterior superior angle is 

 the most anterior portion of the shell. The inferior border is nearly straight, with 

 a well-marked byssal sinus in anterior third, and near the posterior inferior angle 

 the lower border is raised in a slight curve to meet the posterior border, making 

 with it an angle a little less than a right angle. The posterior border is straight 

 and obliquely truncate from above downwards and backwards. The hinge-line is 

 straight and long. The umbones have a broad base where they spring from the 

 valve, are raised above the hinge-line, and are placed in the anterior fifth of the shell. 

 The beaks are incurved, and apparently directed forwards by the angulation of the 

 posterior border, and are non-contiguous. Internal to the umbo is a narrow 

 elongate area, broadest in front, becoming gradually narrower till it vanishes pos- 

 teriorly. Descending from the umbo backwards and downwards to the posterior 

 inferior angle is a sharp ridge, dividing the shell into two unequal portions. Above 

 and posterior to this ridge the shell is so much compressed as to become concave 

 from above downwards ; anterior to the ridge the valve is evenly and regularly 

 curved, except where there is an oblique irregular sinus corresponding to the 

 successive byssal apertures formed during the growth of the shell. 



Interior. — The adductor muscle-scars are shallow, not having left an im- 

 pression in the casts examined. The hinge-plate is narrow in the middle, and 

 broadens behind and in front ; the posterior portion shows two or three subparallel, 

 narrow, elongate teeth or ridges. The anterior portion has not been exposed. The 

 inner surface of the shell is crossed by numerous radiating blunted ridges and 

 grooves, especially well marked near the lower border and close to and in front of 

 the oblique line corresponding to the diagonal ridge. There are one or two distant 

 concentric grooves, more marked in the position of the byssal sinus. The pallial 

 line is entire and remote from the margin. 



Exterior. — The whole of the surface is covered by radiating folds, more marked 

 between the byssal sinus and the oblique ridge, especially so near the lower 

 margin of the valve at this point. The hollow of the dorsal slope has also well- 

 marked radiating folds. These lines are decussated by concentric lines of growth, 

 which are so marked in front and behind the byssal sinus that the radiating folds 

 are decomposed into lines of tubercles. In the neighbourhood of the byssal sinus 

 the concentric lines are bent somewhat irregularly, and are concave downwards. 



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