EXOGYRA. 395 



accept this view, and consider that 0. lunata is a distinct species. They have 

 studied a large series of specimens agreeing with Nilsson's type, and do not find 

 any transitions between 0. lunata and 0. ungulata. After examining a large 

 number of specimens of 0. lunata from Trimingham and comparing them with 

 foreign examples and figures of 0. ungulata, I am led to accept the conclusions of 

 Hennig and Vogel. From the character of the margin of the shell these specimens 

 appear to be fully grown individuals and they are not associated with any 

 examples agreeing with Schlotheim's species. In 0. ungulata the early part of 

 the shell resembles the adult of 0. lunata, but subsequently it becomes more 

 elongate and develops much deeper folds on the posterior as well as on the anterior 

 margin, consequently the valves become much deeper and more nearly equal than 

 in 0. lunata. Since the folding starts at a later stage in 0. lunata than in 0. ungu- 

 lata, it is difficult to see how individuals of the former could ever develop into the 

 latter as has been supposed by some writers. 



Some of the small, sickle-shaped forms of 0. semvptana show some resemblance 

 to 0. lunata, but their attached surface is usually larger and their folds are smaller 

 and more numerous and occur on the posterior as well as the anterior margin. 



Remarks. — Mr. Brydone has found a few examples of 0. lunata, in which the 

 left valve is entirely attached to the surface of echinoids. 



Types. — 0. canaliculata, Sowerby, from Trimingham, is in the British Museum. 

 0. lunata, Nilsson, is from the Alms-sandstone. Hennig states that the figure 

 given by Nilsson does not correctly represent the folds. 



Distribution. — Upper Chalk (zone of Ostrea lunata) of Trimingham, Norfolk. 



Genus— Exogyea, T. Say, 1820. 



(' Amer. Journ. Sci.,' vol. ii, p. 43). 



Exogyea sinuata (Sowerby), 1822. Plate LXI, fig. 13. Text-figs. 194—214. 



1821. Gryph^ea Couloni, M. J. L. Def ranee. Diet. Sci. nat., vol. xix, p. 534. 



— — Dumerilii, Def ranee. Ibid., p. 535. 



1822. — sinuata, J". Sowerby. Min. Couch., vol. iv, p. 43, pl.'cccxxxvi 



(uon Ostrea sinuata, Lamarck, 1819). 



— — aquila, A. Brongrdart. In Cuvier's Ossem. Foss., vol. ii, pp. 



332, 614, pi. ix, fig. 11. 

 1829. — sinuata, J. Phillips. Greol. Yorks., p. 122, pi. ii, fig. 23. 



— Exogyea laevigata, Sowerby. Min. Conch., vol. vi, p. 220, pi. dev, fig. 4. 

 1833. — aquila, A. Goldfuss. Petref. Germ., vol. ii, p. 36, pi. lxxxvii, 



fier. 3. 



