3 



first of these lias been removed to the Filibrauchia and 

 the second to the Eulamellibranchia. 



We see, therefore, that the position of our type is 

 as follows : — 



Class : Lamellibranchia. Ord. : Filibranchia. Sub- 

 order : Pectinacea. Fam, : Pectinidae. Genus: Pecten. 



The American scallop, however, Pecten teniiicostatus, 

 has, according to Drew (1), the gill filaments united by 

 interfilamental vascular junctions, thus forming one 

 exception to the definition of the Filibranchia, and 

 serving to show how insufficient single characters may be 

 in a scheme of classification. 



The genus Pecten is of world-wide distribution, 

 though most of the species are confined to smaller areas, 

 and the habitat extends from the littoral zone down to the 

 450 fathom line and probably further. 



The distribution in time extends from the Cretaceous, 

 and possibly it goes even further back to the Carbon- 

 iferous period. Jackson (3) in his work on the Phylogeny 

 of the Pelecypoda, has shown how this genus is related 

 by the structure of the early nepionic shell to the 

 Aviculidae, and in all probability the fossil Aviculopecten 

 of the Devonian rocks was a connecting link, so that the 

 ancestry of the Pectens cau thus be traced back to Silurian 

 times. 



In addition to Pecten mnxiynus and P. opercnlaris — 

 generally distributed in European seas — the following 

 species are found round the British coast: — Pecten jiusio 

 ('Linne), Pecten vaviiis (Linne), var. purpurea, Jeffreys, 

 and var. nivea, Macgillivray. P. sulcatus (Miiller), P. 

 fragilis (Jeffreys), P. clavatus, var. septemradiafus, Miiller, 

 var. aiha, Jeffreys, and var. dumasi, Payraudeau, P. 

 tigeriniis (Miiller), and var. cnstata, Jeffreys, P. incom- 

 parahili.'! fPissn), P. sfn'atus ('Miiller), P. similis (Laskey), 



