254 THE LAMELLIBRANCHIA 
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classification of the Lamellibranchia, especially by palaeontologists, 
and the subdivisions adopted in this system correspond more or 
less with those based on the structure of the respiratory organs. 
Thus the following are very nearly synonymous terms :— 
Prionodesmacea = Protobranchia + Filibranchia. 
Teleodesmacea = Eulamellibranchia — Anatinacea. 
Anomalodesmacea = Septibranchia + Anatinacea. 
As regards the value of the last order, Septibranchia, in it the 
characteristic organs that have given the name to the whole class 
Lamellibranchia are so profoundly modified, that they differ much 
more from all the other different kinds of gills than the latter differ 
from one another, and therefore, even if the Septibranchia should 
not be placed in contrast to all other Lamellibranchia, they at 
least constitute a group equivalent to the three other groups, 
Protoebranchia, Filibranchia, and Eulamellibranchia. 
Thus the Lamellibranchs are divisible into these four orders. 
It will be remarked that the numerous studies on the organisation 
of Lamellibranchia made since 1891, have shown that there has 
been a progressive evolution in each of these four orders, and that 
consequently such important organs as the heart, kidneys, and 
otocysts may exhibit marked differences in relatively nearly related 
types, and that no strictly pure primitive types have been retained. 
From the point of view of phylogeny the most archaic 
Lamellibranchia are those in which the foot has a “ plantar” ventral 
surface like that of Gastropoda and Pulsellum among the Scaphopoda. 
These archaic forms constitute the Protobranchia (Solenomya, Fig. 
230, Yoldia, Fig. 251, etc.), in which the gonads still retain openings 
into the initial or pericardial portion of the kidneys, and the 
branchial filaments are free and not reflected. From these Proto- 
branchia are derived the Filibranchia, whose branchial filaments 
are reflected, but are still devoid of vascular junctions: these in 
turn have given rise to the Eulamellibranchia, which are more 
specialised in respect of the complication of the ctenidia. Finally, 
eulamellibranchiate forms analogous to the Anatinacea represent 
the source from which the Septibranchia have been derived. 
ORDER 1. Protobranchia. 
These are Lamellibranchia whose distinctive character is the 
possession of gills with flat and non-reflected filaments disposed in 
two rows on opposite sides of the branchial axis (Fig. 206, A, B). 
The mantle is provided with a hypobranchial gland lying on the 
outer side of each gill. The foot has a plantar ventral surface (Fig. 
230, f) and the byssogenous apparatus is but slightly developed. 
The nervous system generally presents a distinct pair of pleural 
