60 



J. E. Hede 



gently concave, runs to the posterior-lower corner, detaching an abruptly sloping 

 field; likewise a faintly, but well marked furrow, directed towards the anterior 

 part of the lower margin. This furrow separates a gently convex anterior part, 

 the breadth of which scarcely amounts one third of whole shell. A slender line 

 originating on the foremost portion of the umbo gives a slightly marked lunula. 



Surface with fine, flattened or gently rounded, slightly elevated, radiating ribs, 

 the interspaces of which have the same width as the ribs. Also traces of a very 

 fine, concentric striation may be distinguished. 



Locality a. 



Stolidotus siluricus n. g. et n. sp. 



Pl. II, fig. 14—17. 



Both valves equal, triangular, rather flattened in the direction of the oro-anal 

 axis, but for the rest moderately convex. The anterior margin almost straight, 

 the posterior rounded, the lower slightly curved. Hinge unknown. Beak a little 

 prominent, bent inwards and a little forwards. It has three strong, radiating, 

 rounded furrows, in the umbonal part all of about the same strength. Towards 

 the lower margin the furrows get wider and shallower, the posterior of them being 

 always fully visible, but the others soon effacing. 



Between the posterior furrow and the concavated posterior field a straight, 

 strong, rounded carina is to be seen. Also at the anterior end, a feeble carina 

 is discernible, marking the limit of a little anterior field, never perfectly observable 

 on our adult specimens, which all have this part depressed and damaged. A juve- 

 nile specimen (Pl. II, fig. 14) on which this strongly sloping anterior field is tole- 

 rably well preserved, makes believe that the valves anteriorly have had an opening 

 under the umbones. In such a case this genus must be closely allied to Pholadomya. 



The surface of the shell is covered with fine, radiating, flattened ribs, separated 

 by narrower, shallow grooves. About 40 such ribs can be counted on the part 

 between the anterior and posterior carina. The ribs on the posterior field seem to 

 be a little stronger than those of the resting part of the shell. Besides these 

 radiating ribs, the shell has very fine, concentric growth -lines. On umbo we also 

 find from 3 to 4 concentric folds, particularly well marked at their anterior end. 

 Young individuals on which this umbonal sculpture dominates, are hereby getting 

 an appearance rather differing from that of the adult specimen. 



Locality a. 



Archinacella dubiosa n. sp. 



Pl. Ill, fig. 10, a, b, and 11. 



Shell cup-shaped, elliptical in outline, widest a little before the center, strongly 

 convex with the greatest height close behind the apex, which is slightly projecting 

 forwards. Young individuals have a more laterally compressed form, with the 



