34 CRETACEOUS ENTOMOSTRACA. 



triangular in outline and less so in section. There is, however, a close agreement 

 with G. sphenoides (Reuss), although our specimen has a greater central convexity, 

 is rather higher and more angular on the back, not quite so straight ventrally, 

 and has a denticulate posterior margin. These differences in the development 

 of details may be of only sexual, if not of varietal value. 



Localitif. — CJtalk-rocJr, Dunstable, Bedfordshire. 



Foreign. — Cretaceous, Strehlen and Gosau. 



. § II. The forms with a postero-ventral wing or spine. 

 3. Oytheropteron alatum (Bosquet). 



Cypeidina alata, Bosquet. Mem. Soc. Roy. Sci. Liege, vol. iv, 1847, p. 369, 



pi. iv, figs. 1 a — d. 

 Cttheee alata, Bosquet. Mem. Comm. Carte geol. Neerlande, vol. ii, 1854, 



p. 117, pi. ix, figs. \Oa—d. 



Length Tl ; height "7 ; thickness 1"2 (with the wings) mm., as stated I.e., p. 370. 

 ,, 1"1 ; ,, *56 ; ,, '8 (with the wings) mm., approximate, as 



calculated. 

 There are many allied forms of Gytheropteron with a more or less expanded 

 sharp ridge or wing projecting from the postero-ventral region of each valve, 

 and the determination of specific and varietal value amongst these various 

 modifications is very diSicult. The following is a list of the most striking of 

 these winged or alate forms. 



Cretaceous. Gypridina alata, Bosquet, 1847 ; (7^yi/iere, 1854. Expanded form. 



Cretaceous. — serratida, Bosquet, 1847; Gythere, 1854. Narrow 



form. 



Tertiary. Gypridina vespertilio, Reuss, 1850 ; Egger, 1858. Expanded form. 



Tertiary. — /iasiaia, Reuss, 1850; Egger, 1858. Expanded form. 



Cretaceous. CytJiere longisp)ina, Bosquet, 1854. Expanded form. 



Cretaceous. — laticristata, Bosquet, 1854. ,, ,, 



Cretaceous. — trigonoptera, Bosquet, 1854. ,, „ 



Cretaceous. — macroptera, Bosquet, 1854. „ ,, 



Tertiary. Gypridina papilio, Egger, 1858. Expanded. 



Tertiary; Cretaceous; Tertiary. Gypridina cornuta, 'Romer, 1838 (narrow); 



Bosquet, 1852 (narrow) ; Reuss, 1855 

 (narrow); Egger, 1858 (broad); Speyer, 

 1863 (broad). 



