Nigerian Eocene Mo I In sea. 79 



wards and diverging from the pallial line ; sculpture at first concentric, then 

 divaricate, consisting of numerous, strong, divergent, erect, equi-distant costae, 

 costal apices commencing from just below the umbonal region, and continuing to 

 the ventral margin, surface of valve closely, concentrically and radially striated, 

 the latter more or less obsolete ; the angulations of the costae become thicker, 

 depressed, less prominent, and more or less imbricating with age ; the growth 

 periods are well marked and nearly equi-distant ; margins serrated. 



DIMENSIONS. 



Small Larger 



example. example 



Height 8 ... 10 



Length ... ... ... ... ... ... 8 ... 10 



Diameter (valves closed) 8 ... 10 



REMARKS. There are only two left valves in the collection to represent this 

 form, which fortunately exhibit well-defined characters. It is distinguished by 

 having thick and very sub-globose valves, an irregular and more or less arched 

 hinge line, a small, wide, and deeply impressed lunule, while the earlier angulate 

 costae are erect and not imbricating, such distinctions separating this shell from 

 other species either recent or fossil. Most species appear to have depressed 

 imbricating costae throughout the valve, whereas the Nigerian shell in its 

 earlier stages has at first fine concentric striations, then a series of erect, angulate 

 costae separated by well-marked furrows, which as age advances become more 

 depressed and assume the imbricating character. A general feature also, for most 

 species is the nearly straight hinge line, which in the present form curves 

 upwards from the posterior margin into a small arch, then descends in a slight 

 curvature anteriorly beneath the lunular cavity. Perhaps among all the known 

 species of this genus, our shell shows closest affinity to Liicina (Strigilla) 

 rigaultiana, Deshayes, 1 from the " Sables Moyens " of France, which belongs to 

 the Bartonian stage of the Eocene series, although according to M. Cossmann 2 

 it flourished throughout Eocene times. That, however, has slightly more equi- 

 lateral, more spreading and less convex valves, the hinge line is much more 

 delicate, quite narrow and nearly horizontal, whilst the angulate costae are 

 continuous from the summits of the shell, whereas the earliest sculpture of the 

 Nigerian valves consists of fine concentric striations. A few Egyptian Eocene 



1 Suppl. Desc. Coq. Foss. Paris, 1858, Vol. I, pi. 47, figs. 28-30, p. 661. 



2 Ann. Soc. R. Mai. Belgique, 1887, Vol. 22, p. 43. 



