familiar genus of the family Pectinidae, the correct 

 position of which amongst Pelecypoda or Lamelli- 

 branchiate Molluscs is not easy to determine. The shells, 

 gills, muscles, mantle, siphons, &c., have all been 

 employed in classifying the Lamellibranchiata, but, so 

 far, without really satisfactory results. The following 

 classification proposed by Pelseneer (7), and founded on 

 the structure of the gills, appears to be the most reliable. 



Protobranchia — Lamellibranchia possessing gills with 

 ilat and non-reflected filaments disposed in two rows on 

 opposite sides of the branchial axis. 



Filibranchia — With gills formed of parallel, 

 ventrally directed, and reflected filaments. The succes- 

 sive filaments are joined together by cilia disposed in 

 *' ciliated discs." 



Eulamellibranchia — In which the gills and branchial 

 filaments are united at regulai"~' intervals by vascular 

 junctions. 



Septibranchia — Dimyarian Lamellibranchs in which 

 the mantle remains fairly open, the gills have disappeared 

 as respiratory organs and have been transformed into a 

 muscular septum dividing the pallial cavity into two 

 chambers. 



Eidewood (12) keeps the first of these orders as it 

 stands, but divides the remaining Lamellibranchs into 

 only two orders, as follows: — 



Ord. I. — Protobranchia (as above). 



Ord. II. — Eleutherorhabda. This is practically the 

 same as the Filibranchia. 



Ord. III. — Synaptorhabda. This includes Pelseneer's 

 two orders, Eulamellibranchia and Septibranchia. 



Thus according to both these classifications, the older 

 group, Pseudolamellibranchia is done away with. This 

 order included the Pectinacea and the Ostraeacea : the 



