340 



Mr. Sharpe does not notice this character, although one of his figures 

 shows a perfect exterior, and it may be that our species belong to a differ- 

 ent genus. At present I shall place them in Ribeiria provisionally ; and 

 should a further examination show that a separation is necessary, I propose 

 Ribeirina as a sub-generic name for their reception. 



In England there appears to be one or two species of this genus in the 

 lower part of the Llandeilo formation. (See Siluria, 3d. edition, pp. 50, 

 549.) 



RIBEIRIA? CALCIFERA. (N. sp.) 



Fig. 326. Fig. 327. 



Fig. 326. Ribeiria? calcifera. er, side view of a specimen with the shell preserved; 

 6, anterior extremity showing the gape and the orifice baneath the beak ; 

 c, dorsal view. 

 " 327. R? longiuscula. Side view. 



Description. Ovate, compressed, narrowed towards the posterior 

 extremity, anterior broadly rounded ; ventral margin more or less convex 

 its whole length, sloping upwards nearly to the dorsal line from about the 

 mid-length backwards ; dorsum from the beaks to the upper posterior 

 angle straight, usually a little concave in the posterior half. The beaks 

 are not prominent, and their position varies from one-sixth to one-fifth the 

 length from the anterior angle. The portion of the dorsum in front of the 

 beaks is nearly straight, situated a little below the line of the portion 

 behind, usually sloping slightly downwards from the orifice to the angle. 

 The dorsum is entire, tnat is to say, it has no hinge, there being only 

 one valve ; it is very narrowly rounded or sub-carinated. The posterior 

 extremity is abruptly truncated at an angle of about 100, the straight 

 margin formed by the truncation being in length about one-fourth or 

 one-third the height of the shell from the ventral margin to the beaks. 

 The gape of the shell extends all round both extremities and the ventral 

 margin ; and it is usually a little less than one line in width. Surface 

 characters unknown, apparently smooth. 



Most of the specimens collected are silicified, and some of them are 

 empty. Several casts of the interior were procured. They show the 

 oblique fissure beneath the beak occasioned by the transverse plate in a 



