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326 
given rise to extremely aberrant types. One of the most extreme of these 
types is the genus Proboscidella. The adults of this genus bear a very 
marked resemblance to the Pelecypod genus Aspergillwm. In the early 
neanic stages Proboscidella resembles an ordinary Productus, from which 
genus the type is known to have descended. Orbiculoidea is a genus 
originating in the Ordovician, and extending through the Mesozoic, The 
first stage is paterina-like, the second resembles Obolella, the third is like 
Schizocrania, and adult growth brings in the characters of Orbiculoideda. 
The geological order of these genera is the same as the ontogenetic order 
of Orbiculoided. 
Ot Orbiculoidea and its allies Beecher (7) says: “The early stages 
of Paleozoic Orbiculoidea have straight hinge-lines and marginal beaks, 
and in the adult stages of the shell the beaks are usually subcentral and 
the growth holoperipheral. This adult discinoid form, which originated 
and was acquired, through the conditions of fixation of the animals, has 
been accelerated in the recent Discinisca so that it appears in a free-swim- 
ming larval stage. Thus a character acquired in adolescent and adult 
stages in a Paleozoic species, through the mechanical conditions of growth, 
appears by acceleration in the larval stages of later forms before the as- 
sumption of the condition of fixation which first produced this character.” 
In the higher genera of the Terebratellidze, the ontogeny recapitulates 
the phylogeny with remarkable fidelity, as pointed out by Beecher (7). 
This example has become Classic, so that it is scarcely necessary to re- 
peat the details. I shall give Beecher’s conclusions in his own words. 
He says: “In each line of progression [the austral and boreal subfami- 
lies| in the Terebrateilidze, the acceleration of the period of reproduction, 
by the influence of environment, threw off genera which do 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 representa- 
tive in a lower genus. Moreover the lower genera are not merely equiva- 
lent 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 at- 
tain higher phases of development, but thicken the shell and cardinal 
process, absorb the deltidial plates, and exhibit all the evidences of 
senility.” 
