12 THE ENTOMOSTRACA OF 



they occur in this argillaceous deposit, we have the opportunity of studying them in 

 all stages of growth. 



We conceive that M. Bosquet's C. Boemeriana cannot be said to differ in any 

 essential point from M. Cornuel's C. sculpta, which latter, if our observations on the 

 stages of growth be correct, is only the unworn adult form of C. pundatula, Roemer ; 

 whilst the figure and description of C. pundatula are sufficiently characteristic of the 

 species which Dr. Reuss and Mr. Williams have named "C. concentrica, to enable us to 

 place the whole under the above-mentioned specific appellation used by M. Rcemer, 

 who found this species in the Hils-clay of Hilse. 



Variety, Virginea. fig. 2 n. 

 Detritus, Charing. Chalk, Gravesend. 



This variety is similar to the above in every respect, except that it is devoid of 

 superficial reticulations, wrinkles, or punctations ; some valves, however, viewed by 

 transmitted light, show faint traces of a reticulated structure. 



Forms of Cythere alHed to the above occur in the Oolites, supplying the majority 

 of species to the Forest-marble, Fuller's Earth, Upper Oolite, &c. They are also 

 plentiful in the Tertiary beds and in the recent state. 



No. 3. Cythere umbonata, Williamson. Tab. II, fig. 3 a — g. 



Cytherina umbonata, Williamson. 1847. Trans. Manchest. Phil. Soc. vol. viii. Memoir 



on some, &c., p. 82, pi. iv, fig. 78. 



INCH. 



Length . . ... . • tV Chalk-marl, Dover. 



Height ...... -g^o Detritus, Charing. 



("inclusive of lateral projections, -^^ Chalk, Norwich and Woolwich. 



Thickness -j 



[exclusive of lateral projections, -^-^ 



Carapace irregular oblong. Valves sub-concentrically punctated, in the young 

 individual depressed, convex in the adult, at all ages bearing a strong, oblique process, 

 spine, or umbo, hollow and conical, placed on the median line, and rather posteriorly. 

 Dorsal edge straight ; ventral somewhat elliptical. Anterior extremity rounded, slightly 

 bearded ; posterior oblique, subacute at its dorsal angle. The hinges are but feebly 

 developed ; the distance between the anterior and posterior hinge is greater than in 

 the generality of the species of this group. 



Dorsal and ventral aspects four-lobed, somewhat cruciform ; posterior irregularly 

 pentagonal. 



This rare and interesting species, first noticed by Mr. Williamson, in the ' Trans- 

 actions of the Manchester Philosophical Society,' as occurring in the Charing Detritus, 

 is found also in the Dover Chalk-marl, and in the Chalk of a few localities. 



