and the Elements in which they live. 161 



we shall, for reasons explained elsewhere*, and from embryolo- 

 gical evidence, place the Pteropoda below the Gasteropoda pro- 

 per, not as an intermediate type between Gasteropoda and Ce- 

 phalopoda ; for the Pteropoda are rather an embryonic type, ex- 

 emplifying, in a permanent form, that stage of development of 

 common Gasteropoda when they are provided with large vibra- 

 cula, and a thin symmetrical shell deciduous in so many of them ; 

 bearing to that state of development of the common Gasteropoda 

 the same relation which the Foraminifera bear to a still earlier pe- 

 riod of their embryonic growth, when the yolk is undergoing its 

 process of gradual successive division, which seems to me to be 

 exemplified in a permanent form in the numerous cells into which 

 the body of Polythalamia or Foraminifera is naturally divided. 

 If this view be correct, the class of Gasteropoda would therefore 

 consist of the three types of Foraminifera, Pteropoda and true 

 Gasteropoda, among which we would place the Heteropoda lowest, 

 and the Pulmonata highest, both on account of their structure, 

 and on the ground of the peculiar mode of development of the 

 Pulmonata. 



The third class is that of the Cephalopoda, which has always been 

 circumscribed within natural limits since the Foraminifera have 

 been removed from it. The position which I ascribe here to the 

 Foraminifera will appear very natural to those who are equally 

 conversant with the succession of fossil types in geological pe- 

 riods, and with embryology, and who know, as we have seen it 

 to be the case also among Radiata, that the higher classes repro- 

 duce in their lower forms types analogous to the lower ones. For 

 the great number of fossil chambered shells, existing in earlier 

 geological periods, is very striking when we compare those old 

 representatives of the class of Cephalopoda with their condition 

 in the present period of the creation, and the natural gradation 

 and analogy between Bryozoa as the lowest type of Acephala with 

 the Foraminifera as the lowest type of Gasteropoda, and the 

 chambered shells of old ages as lower types of Cephalopoda will 

 remind us of similar relations between Polypi as the lowest type 

 of the animal kingdom, the so-called Hydroid Polypi as the low- 

 est type of Acalephse, and Crinoids as the lowest type of Echi- 

 noderms, which are strictly parallel cases in two of the gi'eat types 

 of the animal kingdom. 



If we now start from these modifications in the classification of 

 Mollusca which rest entirely upon anatomical and embryological 

 considerations, to appreciate the relations between the three 

 classes of this type, and the media in which they naturally live, 



* See a paper upon the homologies of Gasteropoda and Acephala with 

 reference to the systematic position of Pteropoda, Foraminifera, Brachiopoda 

 and Brvozoa, read before the American Association, &c. 



