1893.] Ontogeny and Phylogeny in the Brachiopoda. 603 
must be given first consideration. Species, therefore, must be 
based upon surface ornaments, form and color, within certain 
limits, and genera only upon structural features developed 
through a definite series of changes, the results of which are 
permanent in individuals evidently fully adult. 
In each line of progression in the Terebratellide, the accel- 
eration of the period of reproduction, by the influence of 
environment, threw off genera which did not go through the 
complete series of metamorphoses, but are otherwise fully 
adult, and even may show reversional tendencies due to old 
age; so that nearly every stage passed through by the higher 
genera has a fixed representative in a lowergenus. Moreover, 
the lower genera are not merely equivalent to, or in exact 
parallelism with, the early stages of the higher, but they 
express a permanent type of structure, as far as these genera 
are concerned, and after reaching maturity do not show a 
tendency to attain higher phases of development, but thicken 
the shell and cardinal process, absorb the deltidial plates, and 
exhibit all the evidences of senility. 
EXPLANATION OF PLATE. 
Morphogeny of Magellaniinz. 
The figures in the left hand column, A-H, represent the 
stages in the ontogeny of the brachial supports in Magellania, 
one of the highest genera of the family Terebratellide. In the 
right hand column are shown the adult, permanent, generic 
structures, corresponding to the stages of Magellania 
Terebratella passes through all the stages from A to G, Maga- 
sella from A to F, and so on, as far as known for each lower 
genus. ; 
All figures are drawn of approximately the same length, to 
facilitate comparison, in consequence the younger stages are 
much enlarged. 
Fig. A. —Early larval brachiopod, without calcified brachial 
supports, but with circlet of tentacles on lophophore. 
The gwyniform stage. 
Fig. Al.—Gwynia capsula Jeffreys, a morphic equivalent of 
larval stage, figure A. 
