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 
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 phylogeny. In geology the different species of the 
fossil shells of Molluscs are of th2 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 Molluscs 
known to us, is occupied by the Lamp-shells or Spiral-gills 
(Spirobranchia), frequently but inappropriately called Arm- 
footers (Brachiopoda), which have become attached to the 
