40 CRETACEOUS ENTOMOSTRACA. 



Also in its outline, rougbness of surface, and want of wings, it resembles 

 Gytheropteron angulatuw, Brady, ' Ann. Mag. Nat. Hist.,' vol. ix, p. 62, pi. ii, 

 figs. 7 and 8. 



Localities. — Chalh, Whiteabbey, co. Antrim ; Chalk-rock, Dunstable, Bedford- 

 shire. 



8. Cytheuoptekon umbonatum (Williamson). Plate I, figs. 21 — 26. 



Cytheeina UMBOifATA, WUUamson. Mem. Manch. Lit. Phil. Soc, vol. viii, 1847, 



p. 79, pi. iv, fig. 78. 

 CrTHERE UMBONATA, Joues. Moiiogr. Entom. Cret., 1849, p. 12, pi. ii, figs. 3 a — g. 

 — LONGISPINA, Bosquet. Mem. Comm. Neerlande, vol. ii, 1854, p. 96, pi. vi, 



fig. 7. 

 CYTnEEOPTEEON UMBONATUM, Jojzes. Grcol. Mag., 1870, pp. 74 and 76. 

 Cytiieeopteea[on] umbonata[um], Williamson. Mem. Manch. Lit. Phil. Soc, 



ser. 3, vol. v, 1872, p. 136. 

 Cytheee UMBONATA, Marsson, Mittheil. nat. Vor. Neu-Vorpommern uud Eiigen, 



Jahrg. xii, 1880, p. 45, pi. iii, figs. 15 a — c. 



Fig. 21, Length '55 ; thickness '38 mm. 



Fig. 23. „ -6 ; „ -35 „ 



Fig. 24. Height -4 ; „ -4 „ 



Fig. 26. Length -583 ; height -277 „ 



The figures formerly given in the ' Monograph,' 1849, and the present illustra- 

 tions, PI. I, figs. 11 — 13, 23, and 24, indicate such varying proportions in the 

 height and thickness of the valves, the length and sharpness of the postero- 

 ventral processes, and the extent and depth of the mid-dorsal furrow, that we are 

 satisfied this species includes the forms mentioned in the list of synonyms above 

 given. In examining several other specimens in our collections we find further 

 extension of variability, not only in the features mentioned above, but even in the 

 valves losing their subrhomboidal for a more oblong outline. The type (figs. 21 — 

 26) is neatly punctate. 



Of the published forms of this kind, the recent Gytheropteron acutum, Brady, 

 ' Ann. Mag. Nat. Hist.,' ser. 4, vol. iii, 1869, p. 49, pi. viii, figs. 1 — 4, is one of 

 those most nearly approaching G. umbonatum of the Chalk. 



Localities. — Ghalk, Norwich and Woolwich; Chalk-marl, Dover; Detritus, 

 Charing. 



