600 The American Naturalist. [July, 
maturity. Thus, Lingula has 
to (1) Paterina, (2) Obolella, and v. nin of which the first 
two oecur as adult forms in geological formations older than 
any known Lingula. 
Paterina represents the prototype of the Brachiopods. It 
shows no separate stages of growth in the shell, is found in the 
oldest fossiliferous rocks, and corresponds to the embryonic 
shelled condition (protegulum) of the class. 
The genus Orbiculoidea of the Discinidæ 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 Schizocrania-like stage fol- 
lows, 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 pediele and 
freedom of motion. "The discinoid, or discoid, form of Orbicu- 
loidea 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 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 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 
swimming larval stage. Thus,a character acquired in adoles- 
cent and adult stages of Paleozoic species through the mechani- 
cal 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 subfamilies of the Terebratellide undergo 
ice pd series of metamorphoses i in their brachial struc- 
ture. Generic ch ily are generally based upon 
