. Bulletin of the British Museum (Natural History), Geology. ii6 R. H. BATE present species although it can be readily distinguished by its much smaller size and by the distinct median sulcus not present to such an extent here. Progonocythere yonsnahensis sp. nov. (PL 12, figs. 5-14 ; PI. 13, figs. 1-4 ; Text-figs. 16-19) Diagnosis. Small progonocytherid with distinct median sulcus. Eye swelling prominent. Ventro-lateral margin extended into ventral keel. Shell surface punctate. HoLOTYPE. , bed 5, Cayton Bay section. Paratypes. , horizon and locality as above. Description. Car
. Bulletin of the British Museum (Natural History), Geology. ii6 R. H. BATE present species although it can be readily distinguished by its much smaller size and by the distinct median sulcus not present to such an extent here. Progonocythere yonsnahensis sp. nov. (PL 12, figs. 5-14 ; PI. 13, figs. 1-4 ; Text-figs. 16-19) Diagnosis. Small progonocytherid with distinct median sulcus. Eye swelling prominent. Ventro-lateral margin extended into ventral keel. Shell surface punctate. HoLOTYPE. , bed 5, Cayton Bay section. Paratypes. , horizon and locality as above. Description. Carapace small, subquadrate ; elongate in the male dimorph. Greatest length through mid-point; greatest height in the anterior third ; greatest width in the posterior third. As seen in dorsal view a shallow sulcus divides the cara- pace into an anterior and a posterior half. This sulcus is more strongly developed in the female. Ventro-lateral margin strongly convex, extended ventrally into a prominent keel. In a single male specimen there is a second, short keel dorsal to the first and terminating in a small node. Almost directly above this node a second node is situated dorso-medially on each valve of the carapace posterior to the median sulcus. In all other respects this specimen is identical to the others. Dorsal margin strongly convex in the left valve, slightly convex in the right. Anterior rounded with a short, obliquely convex antero-dorsal slope. Posterior triangular : postero- dorsal slope concave ; postero-ventral slope convex. Ventral margin medially incurved. Cardinal angles prominent, the posterior angle being the more strongly angled of the two, anterior angle tending to be broadly rounded. A prominent oval. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London :
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