- 54 - 



present time it is necessary to rely on d'Or'oigny's figizres, 

 despite the fact that occacsionally these figures have heen found. 

 to he more artistic than accurate. .'/"hen compared v/ith the recent 

 form which J3all{l ) has described as a variety of vitrea d'Orhigny's 

 figures indicate that his vitrea is less elongate, more sharply 

 rostrated, more broadly rormded anteriorly and has a more "Dromi- 

 nent escutcheon and posterior carina, a slight posterior sulcation 

 and a fairly well-defined transversely sculptured lunule. On 1-he 

 whole these differences appear to be of an order sufficient to 

 raise Ball's varietal name to Sjoeciiic rank. 



The Bov;den forms are very close to the recent cerata , but 

 cerata of the same size are constantly different in having a less 

 concave posterior dorsal margin, slightly more convex anterior 

 margin, less convex base, less upturned rostrum and lower umbones. 

 Apparently the recent form is quite variable, for a valve rhich 

 is larger than any of the Bowden forms has a more concave post- 

 erior dorsal margin than subvitreamornha . the base and rostrum are 

 similar, but the umbo is proportionately lower. Until the recent 

 species is better understood it seems advisable to consider the 

 Bowden form' as a variety. 



Among Guppy's four cotypes of perle-oida . from the ^liocene 



(1). Bull. Mus, Oompt. Zool., Harvard, vol. 9, p. 126, 1881; 

 idem, vol. 12, pp. 250-251, pi. 8, figs. 12, ISa, 1886. 



