334 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
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 
flat 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 regular 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. . 
Ridewood (12) keeps the first of these orders as it 
stands, but divides the remaining Lamellibranchs into 
only two orders, as follows :— 
Ord. 1.—Protobranchia (as above). 
Ord. IJ.—Eleutherorhabda. This is practically the 
same as the Filibranchia. 
Ord. I1J.—Synaptorhabda. This includes Pelseneer’s 
two orders, Hulamellibranchia and Septibranchia. 
Thus according to both these classifications, the older 
gvroup, Pseudolamellibranchia is done away with. This 
order included the Pectinacea and the Ostraeacea: the 
