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given rise to extremely aberrant types- One of the most extreme of these 

 types is the genus Probos&della. The adults of this genus bear a very 

 marked resemblance to the Peleeypod genus Aspergillum. 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 

 ScJiizocrania, and adult growth brings in the characters of Orbiculoidea. 

 The geological order of these genera is the same as the ontogenetic order 

 of Orbiculoidea. 



Of 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 diseinoid form, which originated 

 and was acquired, through the conditions of fixation of the animals, has 

 been accelerated in the recent Disci nisca 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 Terebratellidre, 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 J in the Terebratellida?. 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." 



