Classification of the Pelecypoda. 157 



superficial in their nature. Thus the muscular impressions 

 were for a long time solely relie I upon ; but an arrangement 

 based on tlie existence of only one or of two adductor 

 muscles, and, in the latter instance, on their relative siz^, 

 proved unsatisfactory. Nor did the change in favour of the 

 presence or absence of siphons and of a pallial sinus yield any 

 better result ; whilst classification based on the form of the 

 foot is now no longer even thought of. 



Neumayr, regarding the matter from a palgeontological 

 point of view, suggested a system based on the hinge cha- 

 racters *, which, however, does not satisfy the stuiant of 

 living forms. 



Fischer, going deeper, made use of the respiratory organs f, 

 but, misled by their external appearance, divided the class 

 into two-gilled and four-gilled groups, ruthlessly sundering 

 closely related families in the process. 



Pelseneer it was who first, in 1889 J, pointed out the true 

 reading of the Pelecypod gill-structure anl founded on t!ie 

 morphology of that organ a classification which, tlioi^li 

 doubtless not final in minor details, promises to be by far the 

 best of any. 



It is extremely interesting to note that Pelseneer fore- 

 stalled by but a very short time an independent worker, 

 Menegaux, vviio had arrived at almost precisely similar con- 

 clusions §. 



Yet another classification has recently been proposed by 

 Prof. C. Grobben ||. In this the external form of the gill is 

 taken as a primary character, whilst the liingi is relied upon 

 as the secondary basis of the arrangement. Tue result, 



* " Zuv Morphologie des Bivalvensclilosses," Sitzb. k. Akad. Wissensch. 

 Wien, Bd. Ixxxviii. Abtli. i. (1884), pp. 38-3-419. The ampliticatiou of 

 which, so far as completed at the time of his death, appeai-ed in the 

 Deiikschr. vol. Iviii. (1891), pp. 701-801. 



t ' Mamxel de Coachyliologie ' (1886-87). 



X " Classilicatiou phylogeuetique des Pelecypodos (communication 

 prelirainaire)," Bull. Scient. France et Belgique, torn. xx. (1889), pp. 27- 

 b'2 ; the complete memoir was published as *' Contribution a I'etude des 

 Lamellibranches," Arch. Biol. (Liege), xi. (1891), pp. 117-312, 18 pis. 



§ In fact his tirst note, " Sur la Branchie des Lamellibranches et sur 

 la comparison avec celle des Scutibranches," Bull. Soc. Philom. ser. viii. 

 torn. i. (1889), pp. 137-141, was elicited by Pelseneer's " preliminary- 

 communication," the full text having b^en sent in as a Thesis, which 

 was afterwards published under the title ' Recherches sur la circulation 

 des Lamellibranches Marins' (4to, Besan9on, 1890). 



II " Beitrage zur Kenntniss des Banes von Caspidaria .... nebst Be- 

 trachtungen liber das System der Lamellibrauchiaten," Arb. Zool. Inst. 

 Univ. VVien, x. ; the latter section also appeared in Zool. Anz. xv. 

 (1892), pp. 371-375, whilst diverse translations are given in Journ. R. 

 Micr. Soc. 1892, p. 772, and Nat. Sci. ii. p. 9. 



