CAMBRO-SILURIAN FOSSILS. 223 



subequal ridges, separated by deep sulci rather less than the 

 ridges in width (three in two lines at six lines from the 

 beak in the middle). Width 1 inch, proportional length of 

 receiving valve y^^^, length of entering valve /jj%, greatest 

 depth of both valves (about the middle) y^^^. 



There are two or three strong imbrications of growth at the 

 margin, but I cannot distinguish any superficial striae in the spe- 

 cimens at my command. In form it is intermediate between the 

 O, inflexa and the O. plana (Pand. sp.), but is more depressed, 

 the beak of the receiving valve not at all incurved, the pseudo- 

 deltidium concave outwardly, and is easily distinguished by its 

 comparatively few, thick, simple ribs. 



Very common in the concretionary limestone of Craig Head 

 near Girvan, Ayrshire; calcareous shales of Colmonel on the 

 Stincher. 



{Col, University of Cambridge.) 



Leptcena tenuicincta (M'Coy). 



Sp. Char. Rotundato-quadrate ; hinge-line as long as the shell 

 is wide ; cardinal angles forming nearly rectangular small flat- 

 tened ears, sides subparallel, obtusely rounded, front wide ; 

 receiving valve subhemispherical, very gibbous, greatest depth 

 a little behind the middle of the shell ; beak large, prominent, 

 obtuse, incurved to the level of the lateral margins ; cardinal 

 area nearly in the plane of the lateral margins, very broad, 

 flat, triangular, height rather more than one-fourth the width ; 

 triangular foramen narrow, closed by a convex pseudodel- 

 tidium ; entering valve very concave, with a mesial septum ex- 

 tending nearly to the margins ; surface of both valves marked 

 with very regular, minute, equal rounded concentric wrinkles, 

 having a slight retral wave in the middle of the front, about 

 twelve in the space of one line at the middle of the shell. 

 Width about 5 lines, proportional length of receiving valve 

 TWo> of entering valve /o%, depth ^%%. 



This beautiful species seems almost identical in size, shape and 

 marking with the Produda tenuicincta (M'Coy) and Leptcena 

 enigma (Vern.), but has a very wide distinctly marked cardinal 

 area. Lest those species should prove to have arese when more 

 numerous and perfect specimens can be examined, I give my old 

 specific name to this undoubted Leptcena. 



Olive schists of Cefn Grugos W. of Llanfyllin, Montgomery- 

 shire. 



(Col. University of Cambridge.) 



