254 THE LAMELL1DRANCH1A 



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, 

 Protobranchia, 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. 231, 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, /) and the byssogenous apparatus is but slightly developed. 

 The nervous system generally presents a distinct pair of pleural 



