CYPRIDEA. 15 



We now find that it occurs abundantly in Tertiary beds at Hamstead Cliflf", in 

 the Isle of Wight. Specimens, young or imperfect, from this locality were 

 described and figured in the ' Geological Survey Memoir on the Isle of Wight,' 

 1856, under the name of Gytherideis unicornis, as a sub-reniform Ostracod, sulcate 

 and tuberculate when young, but with a sharp spine on each valve when adult 

 (see also ' Monogr. Tert. Entom.,' p. 48). Careful examination of a further series 

 of specimens leaves no doubt that it is the same species as that found in the 

 Wealden beds. The Hamstead specimens are not so well preserved as those in 

 the Wealden Clays, nor are they so abundant ; but with the many individuals that 

 have come under our notice, we have been able to match old and young perfect 

 examples from the Tertiary and Wealden formations. 



The Tertiary specimens of this species are very plentiful in a crushed state on 

 the laminae of a dark-grey marl (" D 6 " of the Geol. Survey) in the Lower 

 Hamstead series, Hamstead Cliff. (Mus. Pract. Geol.) 



Description of Ctpeidea spinigeea. 



Length 1 mm. 



Valves obovate, or more generally subtriangularly obovate, varying in the pro- 

 tuberance of the anterior hinge-joint, which is usually strongly marked and 

 angular. Front and hind margins unequally rounded ; the anterior broadly 

 rounded, and with a strong notch and beak; the posterior contracted. Valves 

 slightly convex; edge view narrow-oval, with its outhne broken by the spines. 

 Surface usually strongly punctate all over, but sometimes nearly smooth. A 

 short and blunt but distinct spine is present in mature specimens on the postero- 

 dorsal region of each valve (PI. I, figs. 8 — 11.). In immature specimens (PI. Ill, 

 figs. 1 a, Tertiary, and 1 h, Wealden) the dorsal region has one or more small 

 knobs with transverse sulci, the spine being undeveloped. 



The right valve is the largest, its ventral edge overlapping that of the left 

 valve. (In PI. I, fig. 8, the valves have been modified and misplaced by pressure.) 



Note. — This curious species, or one very much like it, has turned up in a 

 specimen given to me by the late Dr. Mantell as coming from the Oxford Clay of 

 Wiltshire, and also in a piece of the Oxford Clay of Skye, collected by Messrs. 

 Geikie and Young, and there associated with Estheria. If its freshwater habitat 

 in the Hamstead series be a criterion, and if these other specimens prove trust- 

 worthy, it points to more freshwater or estuarine conditions in the Oxfordian 

 series than are usually thought of. — T. R. J. 



