260 



margins nearly parallel ; very convex, especially in- the posterior 

 third. A slight transverse depression occurs near the middle, 

 in front of which can be seen the lucid spots of the muscle- 

 mark, visible externally. These apparently consist of a row of 

 four, and there are probably one or two more in front of them. 

 Surface smooth, Avith a few very faint ridges in the ventral 

 region, and ornamented with scattered shallow pits. Hingement 

 as in pi. i., fig. 2. 



Three examples ; Blue Fullers-earth Clay, Midford. 



24. CYTHERIDEA RETORRIDA,* Sp. n. 



PI. i., figs. 8 a, b, c. 

 , If compared with pi. i., fig. 5, this form is seen to be less 

 symmetrical, smaller, and narrower. The dorsal margin also is much 

 straighter, and the hinge-line is more defined. The posterior 

 and anterior ends are unequally and obliquely rounded, and the 

 ventral margin is much incurved. Edge view, narrow acute- 

 ovate, falling in sharply at the base; end view, ovate. Ornament 

 somewhat similar to that of pi. i., fig. 5 ; the carapace has a 

 dried up, parched, or withered appearance. 



One specimen ; Yellow Fullers-earth Clay, Midford. 



25. CYTHERIDEA PARALLELS, S}^ n. 



PI. i., figs. 9 a, h, c. 



Sub-oblong, with ends obliquely rounded, especially on the 

 postero-ventral margin. Hinge-line long and distinct. Valves 

 very convex ; the left valve overlapping the right in the posterior 

 region, smooth, and ornamented with some shallow, scattered 

 pits. Edge view, almost lanceolate, the greatest fullness in the 

 posterior third ; end view, almost circular. 



Four or five examples from the Blue Fullers-earth Clay,. 

 Midford. 



* Parched. 



