Classification of the Pelecypoda, 1.37 



superficial in their nature. Tims the muscular impressions 

 were for a long time solely relle 1 upon ; but an arrangement 

 based on the 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 palaeontologieal 

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

 racters*, which, however, does not satisfy the student 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 |, pointed out the true 

 reading of the Pelecypod gill-structure and founded on the 

 morphology of that organ a classification wliich, thoa^'i 

 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, 

 M<5negaux, who 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 hiiigj is relied upon 

 as the secondary basis of the arrangement. The result, 



* " Zur Morpholof^e des Bivalvenschlosses," Sitzb. k. Akad. Wissenach. 

 Wien, Bd. Ixxxviii. Abth. i. (1884), pp. 38o-419. The amplification of 

 which, so far as completed at the tima of his death, appeared iu the 

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



t ' Manuel de Conchyliologie ' (1886-87). 



X " Classification phylogenetique des P6l6cypodes (communication 

 prtSlirainaire)," Ball. Scient. France et Belgique, torn. xx. (1889), pp. 21- 

 b'l ; the complete memoir was published as *• Contribution a I'^tude des 

 LameUibranclies," Arch. Biol. (Lie.^'e), xi. (^181)1), pp. 147—312, 18 pis. 



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

 la comparison avec celle des Scutibranches," Bull. Soc. I'hilom. sdr. viii. 

 tom. i. (1889), pp. 1.37-144, was elicited by Pelseneer's " preliminary 

 communication," the full text having been sent in as a Thesis, which 

 was afterwards published under the title ' Recherches sur la circulation 

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



II " Beitriige zur Kenntniss des Baues von Cuspidaria .... nebst Be- 

 ti-achtungeu iiber das System der Lamellibranchiaten," Arb. Zool. Inst. 

 Univ. \Vien, x. ; the latter section also appeared in Zool. Aiiz. xv. 

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

 Micr. tioc. 1892, p. 772, and Nat. 8ci. ii. p. 9. 



