THE PEDIGREE OF MOLLUSCS. 157 



Articulates and Vertebrates, had scarcely got beyond the 

 beginning of their historical development. In subsequent 

 periods, especially in the primary and secondary periods, 

 these higher tribes increased in importance more and more 

 at the expense of Molluscs and Worms, which were no match 

 for them in the struggle for life, and accordingly decreased 

 in number. The still living Molluscs and Worms must be 



o 



considered as only a proportionately small remnant of the 

 vast molluscan fauna, which greatly predominated in the 

 primordial and primary periods over the other tribes. (Com- 

 pare Plate VI. and explanation in the Appendix.) 



No tribe of animals shows more distinctly than do the 

 Molluscs, how very different the value of fossils is in geology 

 and in phjdogeny. In geology the different species of the 

 fossil shells of MoUuscs are of the greatest importance, 

 because they serve as excellent marks whereby to charac- 

 terize the different groups of strata, and to fix their relative 

 ages. As far as relates to the genealogy of Molluscs, 

 however, they are of very little value, because, on the one 

 hand, the shells are parts of quite subordinate morphological 

 importance, and because the actual development of the tribe 

 belongs to the earlier primordial period, from which no 

 distinct fossils have been preserved. If therefore we wish 

 to construct the pedigree of Molluscs, we are mainly de- 

 pendent upon the records of ontogeny and comparative 

 anatomy from which we obtain something like the follow- 

 ing result. (Gen. Morph. ii. Plate VI. pp. 102-116.) 



The lowest stage of the four classes of genuine MoUuses 

 known to us, is occupied by the Lamp-shells or Spiral-gills 

 (Spirobranchia), frequently but inappropriately called Ai-m- 

 footers (Brachiopoda), which have become attached to the 



