594.1 239 



II. 



On a Proposed Classification of the 

 Pelecypoda. 



OF all the groups of Mollusca, the Pelecypoda (or as some 

 would spell it, Pelekypoda) have ever proved the most difficult 

 to classify, being nearly as maddening as the Bryozoa, and one 

 system after another has been tried, only to be laid aside as in- 

 adequate. The shape of the shell, the muscular impressions, first 

 proposed by Lamarck (i), the openings in the mantle (Cuvier, 2), and 

 the presence or absence of siphons (D'Orbigny, 3) have all been 

 abandoned as bases for classification. The methods most in favour 

 at the present day are, perhaps, those based on the characters 

 of the hinge or on the structure of the gills. The former, which, it 

 is interesting to note, formed the principle of one of the earliest 

 arrangements proposed, that of Martini (4), is the one relied upon by 

 Neumayr (5, 6), and now by Dall (7, 8), and naturally is regarded 

 with favour by palaeontologists. 



The gill-structure was first suggested as a possible basis by Ray 

 Lankester (9). Fischer (10) attempted to apply it; but, since he 

 was misled by the superficial appearances of these organs, his method 

 has proved, if possible, less satisfactory than that based on the presence 

 or absence of siphons. Pelseneer (11-13), however, closely followed 

 by Menegaux (14), carried the matter further, and basing his divisions 

 on the intimate structure of the gills, while taking into account the 

 other features of the anatomy, produced a scheme which appeals 

 more especially to the biological student of recent Mollusca. 



Prima facie, one would be inclined to argue that the animal, which 

 moulds the shell to its soft body, was a more important item than che 

 test so formed, and although, as Dr. Dall points out in his pre 

 liminary paper (7), it is not correct to speak, as many do, of the shell 

 as a mere secretion of the mantle — its development being quite as 

 much influenced by the physical forces surrounding it as the mam- 

 malian tooth — at the same time the modifications of the hinge, being 

 so obviously the outcome of the mechanical requirements of the 

 animal to protect itself in its surroundings, can hardly be so reliable 

 a guide to the true affinities as some of the vital organs of the mollusc 

 itself. Who, for instance, acquainted with the shell alone o[ A rctica 



