No. 109.] 69 



subangular along its lateral slopes, closely incurved over that 

 of the opposite valve : dorsal valve much the larger, very 

 prominent in front, obliquely declining towards the beak ; beak 

 depressed, incurved. Surface ornamented by from twenty-five 

 to thirty-three simple subangular plications, seven or eight of 

 which are elevated towards the front of the dorsal valve into a 

 somewhat distinct mesial prominence ; and from six to eight of 

 those on the middle of the ventral valve are depressed so as to 

 form a broad undefined sinus, which scarcely extends beyond 

 the centre of the valve towards the beak, but is prolonged in 

 front, and abruptly bent upwards nearly at right angles to the 

 dorsal valve into a distinct linguiform extension. The plications 

 are marked in front by the usual longitudinal depressed line 

 along the centre of each, and extremely fine regular zigzag lines 

 corresponding to the sharp interlocking, edges of the front and 

 lateral margins of the valves. These fine strias doubtless re- 

 present lines of growth, which have become wholly obsolete on 

 other parts of the shell. 



In this species the two plications bounding the sinus of the ventral valve, 

 and the mesial elevation of the dorsal, sometimes bifurcate towards the 

 beak or middle of the valves, one becoming obsolete on the front. 



It resembles some of the Bohemian forms which are considered varieties 



1 : Rhtnchonella abkupta. 2, 3 : Rhtnchonella vellioata. 



