OLIVER : NEW ZEALAND PELECYPODS. 



187 



with a series greater than was available to Iredale, included quoyi 

 in the description of subtriangulata. His reference of M. lata to 

 its synonymy, and his labels on specimens in the Dominion Museum, 

 prove this. That we are dealing with one variable species, and not 

 two species, is shown by the fact that variations in those characters 

 which are supposed to separate quoyi from subtriangulata may be 

 observed in the same locality. For instance, in shells from the 

 Chatham Islands the angle formed by the dorsal and posterior 

 sides varies through several degrees, while shells from Takapuna 

 vary in the thickness of the shell. While it is thus not practicable 

 to separate a long series of shells from many localities into two 

 species, yet those from the north-east coast between Spirits Bay and 

 Tauranga are usually heavy ventricose shells with the posterior 

 end short and, therefore, the angle made by the dorsal and jposterior 

 sides comparatively small. Shells from Ivaipara and Grisborne 

 southwards, and from the Chatham Islands, are almost invariably 

 of the broad-angled, thin form. If it be convenient to refer to these 

 difierences subtriangulata and quoyi might be used sub-specifically, 

 but in this case quoyi would not have the meaning intended 

 by Iredale, but include besides the greater part, so far as area of 

 distribution goes, of the species subtriangulata. 



AmphideA'^a subti-iangulata pliocenica, n.subsp. 



Specimens from the Pliocene beds at Castleclifi are higher than 

 either of the recent forms, and the angle of the dorsal and posterior 

 sides is intermediate. It is more distinct from the two recent forms 

 than they are from each other, and I here propose for it the sub- 

 specific name pliocenica. Type in the Dominion Museum. Perhaps 



\f^y 



