Jones and Sherhorn — Tertiary Entomostraca. 457 



57. Ctthertjea clathrata, G. 0. Sars. 

 CytJierura clathrata, G. 0. Sars. Brady, Trans. Linn. Soc, vol. xxvi. 1868, p. 446, 

 pi. xxix. figs. 43-46; B. C. and E., Post-Tert. Entom., Pal, Soc. 1874, 

 p. 204, pi. xi. figs. 1-4. 



In this small and interesting form, tlie oval carapace is somewhat 

 sharper behind than before, and more compressed in front than 

 behind. It has the surface ornamented with a strong median ridge, 

 branching freely off towards the margin. The main branches in our 

 specimen keep their entirety, but Dr. G, S. Brady figures individuals 

 in which the branches lose themselves in a rough general reticulation 

 over the surface. From the Weybourn Crag of East Runton ; col- 

 lected by Mr. Clement Eeid, F.G.S. 



58. Cytheropteron triangulare (Reuss). PI, XI. Fig. 16. 



Cythere triangularis, Eeuss, Zeitschr, Deutsch. geol. Ges. vol. vii. 1855, p. 279, 

 pi. X. fig. 3. 

 „ ,, Jones, Monogr. Tert. Entom. 1856, p. 25, pi. vii. fig. 5. 



„ tenuicristata, Reuss, Sitzungsb. k. Ak. Wiss. Wien, vol. lii. 1865, p. [23] 

 plate, fig. 12. 

 This well-marked form has already been well described, as also 

 have several allies, namely, Cytheropteron mucronalatnm, Brady, '"Chal- 

 lenger," 1880, p. 140, pi. xxxiii. fig. 8; C. fene stratum, Brady, Ihid. 

 p. 139, pi. xxxiv. fig. 6 ; and C. sphenoides (Reuss), Denkschr. k. 

 Ak. Wiss. Wien, vol. vii. p. 141, pi. xxvii. fig. 2. 



From the London Clay of Piccadilly ; Sherborn and Chapman Coll. 



59. Cytiierideis gracilis (Eeuss). 



Cylheri7ia gracilis, Eeuss, Haidinger's Nat. Abh. vol. iii. 1850, p. 52, pi. liii. fig. 3. 

 Cytherideis gracilis, Brady, Trans. Zool. Soc, vol. v. 1866, p. 367, pi. Iviii. f. la-d. 



To this neat form, already described by Eeuss and others, the 

 following are more or less allied : — C. arcuata (Bosq.), Tert. Entom., 

 1852, p. 32, pi. i. fig. 14; C. Uthodomoides (Bosq.), Ihid., p. 36, pi. ii. 

 fig. 3 ; G. Bairdia difficilis (Reuss), Sitzungsb. k. Ak. Wiss. Wien, 

 vol. Ivii. 1868, p. 35, pi. v. fig. 7. 



The figure given in the " Fonds de la Mer " (8vo. Paris, 1867-71), 

 livr. 4, 1868, p. 94, pi. xii. figs. 1, 2, of Brady's Aglaia pulchella 

 reminds us of this form. 



We have one small specimen from a Tertiary bed at Colwell Bay. 

 In the closed carapace the right valve is the smallest : it is faintly 

 toothed on the anterior margin. The longitudinal lines on the ventral 

 surface are distinct, though faint. 



Cythkrella, Jones (sub-genus), 1849. 



The members of this genus are separable with difficulty as to 

 their probable specific identities (Monogr. Carbonif Entom., Part I, 

 No. 2, Pal. Soc, 1884, pp. 57-69). For the recognition of the 

 Tertiary forms, with which we are at present concerned, we propose 

 to keep certain types in view, referring our specimens to one or 

 the other of the several groups. 



Group I. — Typified by Cytherella compressa (Miinster), as figured 

 by Egger, ' Ostrak. Ortenburg,' Neues Jahrb., 1858, p. 404, pi. ii. 

 fig. 2, with its wedge-like ends and flat parallel sides. To this we 



