I'ERXA. 



89 



Affinities. — The forms foiuul in the Lower Greensand of England which had 

 been referred to Perna Mullcti by earlier writers were regarded as distinct by 

 Pictet and Campiche, and were named by them Perna Forbesi. They considered 

 that the latter were distinguished by the more prominent folds, the shorter and 

 more deeply concave anterior mai'gin, and the longer posterior wing. An 

 examination of a number of specimens shows that these characters are variable, 

 and I am led to agree with Wollemann in thinking that the forms described by 

 Pictet and Campiche cannot be regarded as more than varieties of a variable 

 species. 



Fio. lo.— Perna Midleti, Desliayes. Lower Greensand (Perna-hed), Sandown. Sedgwick Museum. Left 

 valve X }. (The posterior win},' is broken. Forbes gives a figure of a specimen in which the wing is 

 perfectly preserved.) 



Perna Midleti is tlie type of Fischer's section Mullclia. 



Ti/pe. — From the Neocomian of Veudeuvre. I have not seen the specimens 

 figured by Forbes. 



Distribution.— Perna-hed of Athcrfield and Sandown. Atherfiekl Beds of 

 Haslemere, Peasmarsh, Sbalford, Redhill, Sevenoaks, and Hythe. llythe Beds 

 of Hythe {fide Topley). Lower Greensand of Potton and Upware. Tcalby 

 Limestone (zone of B. bninscicensis) of North Willingham. Speeton Clay of 

 S pee ton. 



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