General Classification of the Pelecrjpoda. 115 



from the protobraucli, so that the difference is merely one of 

 degree of development, svhiie Bernard's comparisons imply 

 a reiationsliip of origin and development for the prodisso- 

 couch and early dissoconch stages. The reasons for the 

 association of these two sections of the Pelecypoda seem to 

 be stronger than the reason for their separation. 



Of course it can be ui-ged against Bernard's order that 

 in the Pectinidae, Spondylid.e, and Plicatnlidce the order of 

 development ot" the teeth is towards the exterior (i. e. the 

 dorsal side), but a foreshadowing of this may be seen in the 

 Xncnlidse and Pectunculidre. 



The separation of Oslrea and Pinna from Avicula on 

 account of the gills is opposed to the results of the researches 

 of Jackson on the Aviculidie and their allies, and of Bernard 

 on the development of the hinge and the general characters 

 of the shell. 



The inclusion of the Trigonacea in the Heterodonta, 

 which is another difference, as the Heterodonta are practi- 

 cally equivalent to the Synaptorhabda, is another point of 

 difference. This position of this family resolves itself into 

 the question of whether the teeth or the gills are taken as 

 being the more important for classification. The arguments 

 on this point were given at the beginning of this paper. 



A third and more important point of difference is th^ 

 inclusion by Ridewood of the Arcidfe with the Trigouiidte 

 and ]\Iytili(lse in the subgroup Mytilacea. This arrangement 

 is opposed to the results of both phylogeny and ontogeny. 

 The types of hinge which these families possess were distinct 

 at any rate in Ordovician times. 



In general basis Bernard^s classification is sounder than 

 Ridewood^s, because it is possible to include in it fossil 

 forms and also because it is not based on the degree of 

 development of a common character. Where the two 

 disagree in detail Bernard's views are supported by other 

 workers and by phylogeny and ontogeny. jMoreover, 

 Bernard's conclusions are the result of the detailed study of 

 ontogeny. 



Literature. 



( i) Barraxde, J. ' Systeme Silurien du centre de la Boheme/ vol. vi, 

 Acephales. 1882. 



(2) Bernakd, p. " Premiere note sur la developpement et la morpho- 



logie de la coquille chez les Lainellibranches (lieterodoutes)," 

 Bull. Soc. geol. de France, (3) t. xxiii. 1895, p. 104. 



(3) . " Deuxiemenote&c. (Taxodontes),"ib. t. xxiv. p. 412. 1893. 



(4.) . "Troisienie note &c. (Anisomvaires),' ib. t. xxiv. p. 412. 



1896. 



8* 



