144 



OSTREA dorsata. 

 TAB. CCCCLXXXIX — /g^s. 1 and 2. 



Spec. Char. Convex, subimbricated; upper valve 

 marked with numerous,, longitudinal^ branched 

 striae ; inner mari^in toothed. 



Syn. Ostrea dorsata^ Deshaijes. 



1 HE convexity of the upper valve of this Oyster appears 

 to have given rise to the name dorsata ; but the form 

 varies much, according- to the form of the substance to 

 which it is attached : the numerous longitudinal im- 

 pressed striae are its peculiar character, and we know of 

 no other fossil Oyster so marked : we have seen a very 

 flat recent one, of a purple brown colour, attached by a 

 large surface, with similar striae. 



Fig. 1 represents a single valve from Hordwell: it is 

 the only one that has fallen under our observation. 



Fig. 2 is an individual from France : it has a ridge 

 along it produced by its having been attached to the 

 stem of an Encrinus, and was sent us with the name of 

 dorsata by Mons. Deshayes. 



OSTREA semiplana. 

 TAB. CCCCLXXXIX.— /g^. 3. 



Spec. Char. Oval, depressed^ largely undulated ; 



in the middle flat ; attachment small. 

 SvN. Ostrea, No. 74, Mantell, Geol. Suss. p. 207. 



t. 25. /. 4. 



One of those Oysters that prefer thin cylindrical subjects 

 for their support, and from which they spread out like 

 leaves : it approaches the plicated division of the Genus 

 by the largeness of its undulations ; but they are not an- 

 gular, and the edge is not deeply sinuated by them. 



Although not abundant, this seems to be a generally 

 diffused fossil of the upper Chalk ; Mr. Mantell obtained 

 it near Lewes. Our specimens are from Norfolk ; the 

 larger, from a chalk-pit close to Norwich belonging to 

 Mr. Barnesj was found at the depth of forty feet. 



