Nigerian Eocene Mollusca. 73 



Family NUCULID^. 

 Nuoula costaeimbHcatis, sp. nov. 



PLATE 6, figs. 10, 11. 



DESCRIPTION. Shell ovately sub-trigonal, small, rather tumid, very 

 inaequilateral, sharply truncated posteriorly ; lunule well defined, cordate, ex- 

 tending to angulation, confluent with ventral margin ; escutcheon well developed, 

 terminating at ventral angle ; denticles strong, slightly angulate and oblique, 

 parallel, about twenty in front and eight behind, enlarging from the 

 umbonal region on the anterior side where they are of minute size ; internal 

 surface nacreous, margins finely crenulated ; sculpture consisting of numerous, 

 equal, sometimes irregular, narrow, closely arranged, imbricating concentric 

 bands crossed by extremely fine, innumerable, radial striations ; umbones 

 rather compressed. 

 DIMENSIONS. 



Height ... ... ... ... ... ... 5 millimetres. 



Length ... ... 7 



Diameter (closed valves) .. ... ... ... 4 



REMARKS. This small shell is chiefly distinguished by its sculpture, 

 the narrowly-banded imbricating costae forming a somewhat rare ornamentation 

 in the surface structure of Nucula, which is more often smooth or furnished 

 with only concentric lineations. Similarly composed costae are, however, 

 observable in N. sulcata of A. Adams * from New Zealand seas, although that 

 species differs in having a rounded posterior margin, a smaller and less 

 defined lunule, fuller umbones, and is besides of larger dimensions. In contour 

 it may be compared with some older Tertiary species such as N. lunulata of 

 Nyst 2 from the Belgian Lutetian desposits (Laekenian), which also possesses a 

 large lunule, a lengthy escutcheon, and a similar postero-ventral angulation ; 

 the denticles are, however, more angulate and more numerous, while there is 

 not the same banded character in the structure of the costae which is so 

 prominent a feature of the Nigerian shell. The species is represented by 

 two specimens, one with both valves in the closed condition, and a single 

 valve exhibiting internal characters. 



OCCURRENCE. Cutting No. i. 

 COLLECTOR. Mr. Kitson. 



1 Proc. Zool. Soc. London, 1856, p. 53. 



- Desc. Coq. Terr. Tert. Belgique : Mini. Cvur. 1843-1845, Vol. 17, pi. 7, fig. 4, p. 231. 



