Palcontological Spcculatioiis. — Gvatacap. 83 
suggestive, but in Scolithus linearis, Arenicolites ivoodi, the 
Myrianites and Nereograptus of Waterville, Kennebec R., 
Maine, we have plentiful evidence of their wide distribution. 
The significance of the trilobites and the striking annehd 
indications cannot be overlooked. Before passing to these the 
thought of Size and Mass, in connection with development, sug- 
gests, with reference to trilobites, an idea of discrimination in 
regard to age. If the development of exoskeletons, hard parts, 
hinges and bony or chitinous coverings, is a mark of advanced 
biogenetic age, then as between two groups of trilobites whose 
relative antiquity is under discussion that group which presents 
the greater surface of such members is the younger. Should 
such a consideration have led a priori to the assumption of the 
greater age of Olenellus as compared with Puradoxides, because 
of the marked pygidial enlargements in the latter over the for- 
mer? For in Olenellus the lateral lobes of the pygidium are 
absent, the axis alone remaining, while slight pygidial wings 
are incipient in Paradoxides. The growth of pygidia is notice- 
able in the trilobites of the higher Cambrian series, as in Pty- 
choparia (Conocephalites), Dicellocephalus &c. 
In Agnostus and Microdiscns the dual development of 
head and tail is certainly conspicuous, but their category of 
growth and relationship is entirely different from the mul- 
tipleural forms represented in Paradoxides and Olenellus. 
Olenoides shows also a uniform growth of head and tail. The 
impression given by a study of these early trilobites is that they 
were separated in habits of life into two groups, a rapidly mov- 
ing natatory group like Olenellus, Protypus and Paradoxides 
with well developed head shields from mechanical reaction 
against pressure, and a sedentary group in which glabella and 
pygidium were more evenly related. And that between these 
extremes Conocoryphe, Ptychoparia, Ellipsocephalus, Agrau- 
los, etc., presented a less determined phase of activity or rest. 
These considerations, of course, have no reference to their mor- 
phological affinities. There is certainly a definable relation be- 
tween activity and cellular deposition of hard parts. Embry- 
ology proves that. The early stages of many sessile organisms, 
which develop in their maturity hard parts and more or less 
significant exoskeletons, are marked by great motility and ex- 
cursive power. Such are the free-swimming larvai of the AIol- 
