31 G COMPARATIVE ANATOMY. 



I submit the following sketch of the classification of the group, 

 and would remark that many of the older views took note of varia- 

 tions, which would still further separate the divisions, and especially 

 the smaller ones, of the group. 



I. Placophora. 



. Chiton, Cryptocliiton. 



II. Conchifera.* 



Lamellibranchiata. 



Asiplioni'a. 



Ostrea, Anoinia, Pccten, Mytilus, Area, Anodonta, Unio. 

 Siphoniata. 



Chama, Cardium, Cyclas, Venus, Tellina, Mactra, Sulen, rholas, 

 Teredo. 



Scaphopoda.f 



Dentalium. 



Gastropoda.^ 



Prosobranchiata. 

 Chiastoneura. 



Zeugobranchia. 



Fissurella, Haliotis. 

 Anisobrancliia. 



Patella, Trochus, Littorina, Cyclostoma, Rissoa, Paludina, 

 Turritclla. 

 Orthoneura. 



Nerita, Janthina, Valvata, Sigaretus, Marsenia, Cypraea, 

 Cerithium, Strombus, Pterocera, Dolium, Cassis, Tri- 

 toniiun, Voluta, Harpa, Bueciuum, Nassa, Purpura, 

 Murex. 

 Heteropoda.§ 



Atlanta, Carinaria, Pterotracliea. 

 Opisthobranchiata. 

 Tectibranchiata. 



Bulla, Aplysia, Pleurobninclms. 

 Nudibranchiata. 



Tritonia, Polycera, Aeolidia, Phyllirlioe, Doris, Phyllidia, 

 Pleuropbylliilia. 

 Sacoglossa. 



Elysia, Limapontia, Placobranclms. 

 Pulmonata.|| 



* What led me most to unite all tho Mollusca, with the execptiou of the Chitouida 1 , 

 into one great division, to which I have given the name Conchifera, was the considera- 

 tion that we must recognise the great significance of the shell as affecting the whole 

 organisation of these animals. But although the Placophora are thereby sharply 

 marked off from the rest, I do not see that there is any sufficient reason for removing 

 them altogether from the Molluscau phylum, for it is possible to make out in them 

 many points in which they agree with, and are consequently allied to, the Conchifera. 

 I regard the Placophora as the remnant of a division, the forms of which were allied 

 to the Soleuogastres (p. 127) on the one hand, and on the other were the predecessors 

 of the Conchifera. 



f The Scaphopoda form a division which is allied to the Lamellibranchiata as well 

 as to the Gastropoda ; but they must not be regarded as a mere intermediate link. 



X The Zcugobranchia are, in many points, the oldest of the Gastropoda. 



§ I regard the Heteropoda as an order which has branched off from the Proso- 

 branchiata, and is closely allied to the Orthoneura; but which has developed special 

 characters which are not equal in value to those of the Orthoneura. 



II The organisation of the two divisions of the Pulmonata does not seem to me to 

 be so markedly divergent as to make them of equal value with the two other orders of 

 the Gastropoda. We cannot as yet form a definite opinion as to many of the genera, 

 e.g. Onchidium, of the Nephropneusta. 



