PARALLELISM. 177 



It shows no separate stages of growth in the shell, is 

 found in the oldest fossilifarous rocks, and corresponds 

 to the embryonic shelled condition (protegulum) of 

 other brachiopods. 



The genus Orbiculoidea of the Discinidffi first ap- 

 pears in the Ordovician and continues through the 

 Mesozoic. The early stages in the ontogeny of an in- 

 dividual are as in Lingula, first a paterina stage, fol- 

 lowed by an obolella stage. Then from the mechan- 

 ical conditions of growth a Schizocrania-like stage 

 follows, and completed 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 of Orbiculoidea and Discinisca among the 

 brachiopods, and Anomia among pelecypods, is deter- 

 mined by the horizontal position of the valves, which 

 are attached to an object of support by a more or less 

 flexible, very short organ, a pedicle or byssus, without 

 calcareous cementation. This mode of growth is char- 

 acteristic 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 subcen- 

 tral and the growth holoperipheral. This adult disci- 

 noid form, which originated and was acquired through 

 the conditions of fixation of the animal, has been ac- 

 celerated in the recent Discinisca, so that it appears in 

 a free-swimming larval stage. Thus, a character ac- 

 quired in adolescent and adult stages of Paleozoic spe- 

 cies through the mechanical conditions of growth, ap- 

 pears by acceleration in larval stages of later forms 

 before the assumption of the condition of fixation which 

 first produced this character. 



