Dr. J. Allan Thomson — On the Thecidiince. 



463 



species is unknown, but the presence within the umbo of the ventral 

 valve of a " cup-shaped cavity divided longitudinally by a median 

 septum " seems to connect it with Lacazella. All the Tertiary forms 

 of Europe to the literature of which I have access belong also to 

 Lacazella, viz. L. Adamsi, McDonald, from the Miocene of Malta, 

 L. testudinarium, Michelotti, from the Miocene of Italy, and 

 L. latdorjiense, Davidson, from the Oligocene of Germany, and the 

 genus has also European Cretaceous representatives. 



ThECIDELLINA HED1EYI, Sp. nOV. 



Description. — Shell attached by the beak of the ventral valve, 

 irregular in shape but in dorsal aspect rounded trigonal with a nearly 

 straight front, greatest width near the anterior margin. Dorsal 

 valve nearly flat, irregularly waved, raised near the umbo. Ventral 

 valve strongly convex, oyster-like ;" commissures practically plane. 



FlG. 1. — Thecidellina hedleyi, Thomson, Kakanui, New Zealand (enlarged 

 10 diameters), (a) Holotype, dorsal view ; (b) paratype, interior of dorsal 

 valve, ventral view. 



Hinge-line straight, moderately broad, beak sharp, without foramen, 

 area triangular, flat. Interior of dorsal valve : socket ridges 

 converging slightly posteriorly and uniting mesially to form a process 

 which is subrectangular and projects posteriorly considerably beyond 

 the hinge-line, and is concave anteriorly. In front of the socket 

 ridges there are two spurs pointing laterally inwards, probably the 

 remains of a broken bridge. The. margin is granulated, and is 

 produced into a median septum, low anteriorly, rising posteriorly, 

 and reaching a little beyond the middle of the shell. From this 

 septum two lamellae probably curved laterally forwards on each side, 

 but are now disconnected in the specimen available. The granulations 

 do not continue from the margin to the septum. Interior of ventral 

 valve unknown. 



