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ONTOGENY AND PHYLOGENY IN BRACHIOPODA 287 



adult forms in geological formations older than any known 

 Lingula. 



Paterina [= Iphidea] represents the prototype of the brachi- I 

 opods. It shows no separate stages of growth in the shell, j 

 is found in the oldest fossil if erous rocks, and corresponds to ' 

 the embryonic shelled condition (protegulum) of the class. 



The genus Orbiculoidea of the Discinidse first appears in 

 the Ordovician and continues through the Mesozoic. The 

 early stages in the ontogeny of an individual are, as in 

 Lingula, first a Paterina stage, followed by an Obolella stage. 

 Then, from the mechanical conditions of growth, a Schizo- 

 crania-likQ stage follows, and complete growth results in 

 Orbiculoidea. 



The elongate form of the shell in Lingula as well as in 

 many other genera is determined by the length of the pedicle 

 and freedom of motion. The discinoid, or discoid, form of 

 Orbiculoidea and Discinisca among the brachiopods, and 

 Anomia among pelecypods, is determined by the horizontal 

 position of the valves, which are attached to an object of sup- 

 port by a more or less flexible," very short organ, a pedicle 

 or byssus, without calcareous cementation. This mode of 

 growth is characteristic of all the discinoid genera, but, as 

 already shown, 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 sub-central and the 

 growth holoperipheral. This adult discinoid form, which 

 originated and was acquired through the conditions of fixa- 

 tion of the animals, has been accelerated in the recent Dis- 

 cinisca so that it appears in a free-swimming larval stage. 

 Thus, a character acquired in adolescent and adult stages 

 of Paleozoic species, through the mechanical conditions of 

 growth, appears by acceleration in larval stages of later forms 

 before the assumption of the condition of fixation which first 

 produced this character. 



The two chief sub-families of the Terebratellidse undergo 

 complicated series of metamorphoses in their brachial struc- 



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