CYTHEROPTERON CONCENTRICUM. 31 



with parallel wrinkles and furrows. The postero-ventral region of each valve is 

 in some cases produced laterally into a narrow wing or a sharp spike. 



§ I. The forms with full and rounded ventral region, either smooth, marked with 



riblets, or ridged. 



1. Cytheropteron CONCENTRICUM {Betiss). Plate I, figs. 5 — 10; Plate IV, fig. 19. 



Cttheeina concenteica, Eeuss. Verstein. bohm. Kreideform., ii Abtheil., 1846, 



pp. 104 and 105, pi. xxiv, figs. 22 a — e. 

 CiTHEEE SCTJIPTA, Cornuel} Mem. Soc. geol. France, ser. 2, vol. i, pt. 1, 1846, 



p. 201, pi. viii, figs. 20—23; and vol. iii, pt. 1, 

 1848, p. 244. 

 Cttheeina concenteica, Williamson. Trans. Manchester Lit. Phil. Soc, vol. viii, 



1847, p. 79, pi. iv, fig. 77. 

 Ctpeidina Rcemeeiana, Bosquet. Mem. Soc. Eoy. Sci. Liege, vol. iv, 1847, 



p. 362, pi. ii, figs. 2 a—f. 

 Ctthere punctatula, Jones (non Rimer). Monogr. Entom. Cret., 1849, p. 11, 



pi. i, figs. 2 a — m (fig. 2 w, var.). 



— concenteica, Bosquet. Mem. Com. Carte geol. Neerlande, vol. ii, 1854, 



p. 81, pi. viii, figs, a — d. 



— — Jones. G-eol. Mag., 1870, pp. 74 and 76. 



— — Williamson. Trans. Manchester Lit. Phil. Soc, ser. 8, 



vol. V, 1872, p. 136. 



— — Beuss. Elbthalgeb. Sachsen, 1874, p. 144, pi. xxvii, figs. 



1 a — c. 



— — Kafka. Crustaceen bohm. Kreideformation, 1887, p. 14, 



fig. 27. 



PI. I, fig. 5. Length "66 ; height '44 mm. 



PI. I, fig. 6. „ -61; „ -33 „ 



PI. I, fig. 7. „ -861 ; „ -351 „ 



PI. I, fig. 10. Height -55 ; thickness '833 mm. 



PI. IV, fig. 19. Length '83 ; height '56 „ 



The late M. Bosquet, of Maastricht, having carefully compared specimens of 

 Romer's G. 'punctatula with G. concentrica, Reuss, decided that the former is the 

 young state of G. Hilseana, Romer (see ' Mem. Comm. Neerlande,' vol. ii, p. 82). 

 " G. punctatula, Romer," is therefore omitted from the synonymy. 



Numerous gradations are observable in the many individual valves, whether 

 from one or from different localities, as to the superficial ornament of this some- 

 what variable and yet very distinct species. The very delicate concentric 

 reticulation, with or without spinous mesh-walls, becomes coarser and coarser 



^ M. Cornuel had mentioned hia Neocomian species already in the ' Bullet. Soc. g^ol. France,' 

 ser. 2, vol. ii, p. 52, 1844. 



