42 CRETACEOUS ENTOMOSTRACA. 



This is larger and more convex than the type, with long and sharp ventral 

 spines, themselves delicately spinose, and spreading out wide (PI. Ill, fig. 12, and 

 PI. IV, fig. 31), and sometimes nearly straight (fig. 13). The mid-dorsal furrow 

 is generally stronger than shown in figs. 11 and 12, and occasionally is repeated 

 to some extent in the anterior region, so as to give rise to a small antero-ventral 

 tubercle. In one case even a feeble antero-dorsal spine is present. The surface 

 is punctate, with the meshes, in several specimens, becoming spinose, so that the 

 whole surface is prickly. The front margin is strongly denticulate. 



The figs. 15 a — c by which Dr. Marsson illustrates G. umhonata^ Williamson, 

 evidently belong to the same variety as our PI. Ill, figs. 11 — 13, and PI. IV, figs. 

 30 and 31, although in the latter the spines are longer, the valves more prickly, 

 and slightly modified by the subsidiary furrow in front. 



Localities. — Ghalk, Horstead, and chalk-flint in gravel at Mitcham, Surrey 

 (collected by Dr. G. C. Wallich). 



Foreign. — Chalk, Isle of Riigen. 



9. Cytheroptbhon Sherboeni, sp. nov. Plate I, figs. 33 and 34; Plate IV, figs. 



20 and 21. 



PI. I, fig. 33. Length 725 ; height -375 ; thickness "35 mm. 



PI. IV, fig. 20. „ -8; „ -43; „ -53 „ 



Several specimens from Horstead, Norfolk, although very tumid over the 

 ventral border, have no postero-ventral spine ; some, however, have a trace of a 

 ventral ridge ending with a posterior angle. The surface has a variable concentric 

 punctation, with the meshes in some instances becoming spinose, and thus making 

 five or six concentric rows of small prickles (figs. 33 and 34). The valves are nearly 

 oblong in outline ; obliquely rounded and denticulate in front, and blunt or more 

 or less angular behind. The mid-dorsal sulcus is present, and ends in a minute 

 subcentral pit. Both in front and behind, the valve is depressed along the margin, 

 sometimes considerably below the convex body of the valve. 



The Post-tertiary Cytheropteron complanatum, Brady and Crosskey, ' Geol. 

 Mag.,' 1871, p. 65, pi. ii, figs. 3 and 4, from Canada, has a distant resemblance to 

 this form. 



The concentric punctation is, to some extent, comparable with that of Cythere 

 vesiculosa, Bosquet, ' Mem. Comm. Neerlande,' vol. ii, p. 94, pi. vi, figs. 2 a — d, 

 but the arching of the semicircular lines is reversed, being towards the dorsal 

 instead of the ventral region as in our specimens. 



