378 CRETACEOUS LAMELLIBRANCHIA. 



1905. Exogyra lateralis, T. Wegner. Zeitschr. d. deutsch. geol. G-esellsch., 



vol. lvii, p. 184. 

 ? 1907. GrRYPH.£OSTREA vomer, 8. Welter. Cret. Pal. New Jersey, p. 455, pi. xliv, 



figs. 6 — 11. 

 1909. Ostrea (Exogyra) lateralis, W. Bogota. Bull. Internal Acad. Sci. 



Cracovie (1909), 2, 

 p. 691. 

 1911. Exogyra lateralis, Fritsch. Op. cit., Korycan. Schicbt., p. 46, fig. 210. 



K. Vocjel von Falckenstein. Zeitschr. d. deutsch. geol. 



Gesellsch., vol. lxii, 

 p. 561. 



Description. — Shell generally rather small, usually higher than long, very 

 inequivalve. 



Left valve very convex or inflated ; the greatest convexity is between the 

 umbo and the postero-ventral extremity, and gives in some cases the appearance 

 of a much rounded carina, behind which the valve is either flattened or slightly 

 concave ; sometimes there is a posterior Aving-like expansion. The ventral 

 margin is rounded ; the lower part of the posterior margin is often slightly 

 concave, ending above in a point. Umbo usually much incurved, and directed 

 posteriorly. The ligament pit usually curves posteriorly. This valve is attached 

 posteriorly to the umbo ; the size, shape, and direction of the attached surface 

 vary in different individuals, and affect the form of the shell in the usual 

 way. The surface of the shell shows growth-lines, and in some specimens 

 there are numerous, fine, somewhat irregular ridges running parallel to the 

 margin of the valve. In the interior of the valve a few widely separated growth- 

 ridges occur. 



Right valve operculiform, nearly flat, or concave or undulating. Umbo very 

 small, curving spirally forwards. Surface with widely separated ridges running 

 parallel to the margin of the valve. 



Affinities. — The Albian and Cenomanian examples of this species were regarded 

 by Coquand as distinct from those found in the Upper Chalk and named 0. lateralis 

 by Mlsson; but d'Orbigny, Geinitz, Peron, Jukes-Browne, and Hennig have 

 shown that the forms named lateralis cannot be separated from those which occur 

 in earlier beds {canaliculata). Peron states that on the Continent this species 

 ranges without interruption through all the stages of the Cretaceous, from the 

 Aptian to the Danian; he also notes its wide geographical distribution. This 

 species belongs to Conrad's " genus " Gryphaeostrea ; it has been referred to 

 Gryphsea by some writers, but more usually to Exogyra ; the left valve, except for 

 the attached surface, is often grypheate in form, while the right valve shows some 

 resemblance to Exogyra. Pictet and Campiche consider that O. canaliculata is 

 closely related to Exogyra conlca ; but it seems more probable that it is allied to 



