368 CRETACEOUS LAMELLIBRANCHIA. 



of the valves the left valve becomes convex ; if the axis is more or less parallel to 

 the plane of the valves and to the hinge-line the left valve is less convex ; and if 

 in such cases the attached surface is mainly in front of the umbo then the anterior 

 part of the shell becomes elongated, whereas, when the attached surface is mainly- 

 posterior to the umbo the posterior part of the shell become elongated and the 

 shell more inequilateral ; if the long axis of the attached surface extends from the 

 umbo in a postero-ventral direction the shell becomes oblique and very inequi- 

 lateral. Occasionally the left valve shows two surfaces of attachment, one at the 

 umbo, the other at some distance from it. Since the direction of growth depends 

 mainly on the position of the attached surface, it is obvious that the relative length 

 and height will be determined in the same way. 



The surface of the free part of the left valve is smooth except for growth- 

 lines. The form and direction of the umbo, area and ligament-pit depend on the 

 mode of attachment. The area is either nearly parallel to the attached surface or 

 slopes gently from the hinge-line to the umbo, and consequently in the grypheate 

 forms it eventually becomes more or less nearly perpendicular to the plane of the 

 valves of the adult shell, but in completely attached forms the area and ligament- 

 pit remain nearly parallel to the plane of the valves ; between these two extremes 

 every gradation can be traced. The ligament-pit and the umbo may curve 

 forwards or backwards, or be perpendicular to the hinge-line. Small, irregular, 

 rounded ridges or crenulations, more or less nearly at right angles to the margin 

 of the valve, are seen on the inside of the shell on each side of the area, but are 

 most developed on the posterior side. The adductor impression is rounded or 

 slightly oval. 



The right valve in the grypheate form with a very small attached surface is 

 almost entirely concave or sometimes nearly flat. The part of the right valve 

 which corresponds to the attached part of the left valve is more or less convex,, 

 and reproduces the detailed markings as well as the general form of the attached 

 surface ; when the latter is flat the corresponding part of the right valve is only 

 slightly convex, and its convexity increases with that of the attached surface. As 

 soon as the left valve grows free from the attached body the corresponding part of 

 the right valve becomes concave and grows upwards at a considerable angle with 

 the earlier convex part. When the entire left valve is attached the whole of the 

 right valve is slightly convex and there is no upward growth of the marginal part. 

 When the outer layer of the right valve is well preserved fine thread-like radial 

 ribs are seen ; they are widely separated and either straight or slightly irregular. 

 The inner margin near the area has small ridges like those of the left valve. 



Affinities. — The study of a large series of specimens shows clearly that Osfrea 

 tiippop odium, Nilsson, is only a form of 0. vesicularis in which the entire or almost 

 the entire surface of the left valve is attached, and consequently that valve is 



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