Nigerian Eocene Mollusca. 95 



in front of the other and rather more elevated, such as is observable in 

 Corbiila gallica, C. exarata, etc., from the European Eocene deposits. In its 

 projecting character the left chondrophore resembles that present in Mya, but 

 instead of merging with the postero-dorsal margin as in that genus it radiates 

 outwards, with the other extended elements of the hinge -plate, from a point 

 immediately beneath the umbo. A similarly projecting chondrophore is also 

 apparent in members of the Corbulidae, otherwise such shells as well as i\Iya 

 are quite distinct from the present genus. 



In Raeta the chondrophores are of similar construction and position in 

 both valves ; it has, moreover, cardinal teeth, and a siphonal inflection which 

 is deep, angulate, and sloping downwards from the posterior adductor 

 scar, being nearly parallel with the ventral margin. The adductor scar 

 impressions are of the same design as those of the African shell, while the 

 umbones occupy a more approximate position and are on the same 

 plane. 



The shell has similar dorso-marginal sulci as that of Raeta, which it is 

 considered would be for the reception of ligament and therefore amphidetic, 

 as expressed in Neumayr's 1 terminology, as opposed to opisthodetic, when 

 the ligament is entirely behind the umbones. 



Again, there are no paired laminae to receive the lateral teeth as in the 

 genus Mactra ; these are also absent in Raeta. These and the other 

 distinctions already referred to have been considered of sufficient importance 

 for regarding Raetomya under the new family designation of Raetomyidae. 

 In like manner it has been suggested that Raeta, from its differences to 

 Mactra, in contour, its unequal adductor scars, and the possession of a large 

 angulate pallial sinus, should be placed under the family name of Raetidae 

 instead of Mactridae. 



This genus Raetomya appears to be restricted to African Eocene deposits, 

 whereas Raeta ranges from Eocene times through the later tertiaries on to 

 the seas of the present day, being known from the rocks of Peru, the United 

 States, and Switzerland. 



i Denkschr. k, Akad, Wiss. Wien, Vol. 58, 1891, p. 711, 



