- 181 - 



0. trig'onalis Go rad (l) of the same size, but tri pen alls is thick- 

 er, with more definite oonoentric laminae and a wider ligament 



(1). Jonrad, i'. A., Proc. Acad. L-at. Sci., Philadelphia, vol. 

 7, p. 259, 1855. 



area. 



Gaht (2) suggested the ici entity of his 0, oerrosensis . from 



f2). loc. cit., p. 106. 



the L'io^ene (?) of Oerros Island, off Iov7er California, with the 

 undescrihed , but figured, recent 0. "-alius Valenciennes, but stat- 

 ed that cerrosensis is not more than half as large as the recent 

 f or-- . He also noted the occxirrence of an oj-ster in the late 

 Tertiary of Payta, '^eru, which he considered identical vath gallus ; 

 in fact, later in the sa'^ie year in the Journal of ^oncholop-y he 

 called the Peruvian form p-al lus . Reeve apparently v;as the first 

 to recognize the identity of .callus with the previou.sly described 

 megodon . If the above synonymy is accepted, this species is, as 

 Dall has stated, one of those forms which formerly v/as common to 



the Antillean and Pacific regions, but after the senaration of 

 the tv.'o provinces persisted only on the Pacific side, ivamples 

 of such distribution are frequently encourtere 1 , not only among 



