144 The American Geologist. MHrch, iswi 
(Uvided the class into four orders, the Afniwitd. Nrofn-mnfa, Pio- 
fremnta and Tdotrciiiata. In the Atninntn^ the pedicle passes 
••freeh' from between the two valves, the opening being more or 
less shared b_y both," while in the Nrotmnata, the pedicle opening 
is restricted to one valve, the ventral, "remaining open in primi- 
tive mature forms [_Trfniattdte], becoming enclosed in secondarj' 
forms during nealogic stages IDiseinidw^, and in derived types 
enclosed in early nealogic or nepionic stages [Acrofrcfida-]." In 
the Frotrcinatd, there is a deltidium (the pseudo-deltidium of 
authors), which, in the earliest primordial, appears to begin as a 
short plate covering but a small portion of the delthyrium {Kntor- 
giiia ciiigulata after Walcott. according to Beecher). This plate 
rapidly attains its full growth, closing the entire delthyrium of 
the ventral valve, as in Clitamhouifis and Billinysdla, while in 
the Orthidcc it is developed only during nepionic or nealogic 
growth. The delthyrium in the Trlofrmiafa is without any trace of 
covering during nepionic growth, but during the succeeding stages 
there grow out from the walls of the former two plates (the del- 
tidial plates) which usually meet medially, and may become 
anchylosed. 
It is remarkable that three of the four types of pedicle openings 
should appear in the earliest known horizon of the primordial, yet 
fundamental structures in other classes of organisms have devel- 
oped with equal rapidity. Prof. Hyatt" saj's "the acknowledged 
sudden appearance of the larger number of all the distinct types 
of invertebrata in the paleozoic, and of the greater number of all 
existing and fossil types before the expiration of the paleozoic 
time, speak strongl}' for the quicker evolution of forms in the 
paleozoic and indicate a general law of evolution. This, we think, 
can be formulated as follows: Types are evolved more quickly 
and exhibit greater structural differences between genetic groups of 
the same stock while still near the point of origin, than the)' do 
subsequently. The variations or differences may take place 
quickly in the fundamental structural characteristics, and even 
the embr3'os may become different when in the earliest period, 
but subsequently onlj' more superficial structures become subject 
to great variations." All the fundamental structures, as the 
four types of pedicle openings and the various calcareous sup- 
ports of the brachia. were in existence during the Trenton period 
of the Lower Silurian. 
