' Palcontological Speculations. — Gratacap. 221 
However that might he the primordial trilohites seem, as 
the earliest crustaceans, a good illustration of the influence 
of environment. They lived in shallow seas, along sandy 
shores, and occasionally in muddy and calcareous slimes. But 
in their motion over the sea hottoms or their occasional ex- 
cursions into deeper water they constantly encountered an op- 
posing pressure and the effect of mechanical resistance, in 
accordance with the principles of histological modifications 
enunciated and established by Ryder, would have hastened the 
growth of the trilobitic head-shield. Ryder has written of 
the hard exoskeleton of chelonians and armadilloes," "the or- 
igin of dermal ossifications is to my mind rationally explained 
by supposing the bioplasm of each dermal cell as sensitive and 
irritable to rude or violent external impacts, which, oft re- 
peated, act as stimuli of growth force, determining certain 
tracts of these cells as the nidus within which osseous par- 
ticles eventually appear as nuclei of the future defensive der- 
mal bony system." The same theorem has received discus- 
sion in Prof. Cope's well known address on "Relation of An- 
imal Motion tO' Animal Evolution," and a capital illustration 
of one phase of it is found in the strengthened lateral teeth 
in the plates of Chitons exposed to the mechanical violence of 
waves. ( See Pilsbry. ) 
The chamge from a creeping habit to a swimming one 
would have, in accordance with this law, developed a harden- 
ing chitinous envelope over the head parts of the metamor- 
phosing annelid, wherever the stimulating effects of pressure, 
against water, sand, or mud, were felt. Environment and 
habit, on the assumption of an annelidan origin, must have had 
a sensible influence in j^roducing primarily the crescentic 
shield-shape of the trilobites glabella. 
Natural Selection may have played some part in the rapid 
extermination of DiccUoccphahis, Aglaspis and Crcpiccphalus. 
They are found with an extremely limited range, and, in the 
appreciable vertical distribution of Dicclloccphahts, the occur- 
rences are sporadic. Broad expanded shields or blunt shoul- 
dered ones would have generally proved unfavorable to con- 
tinued existence along a stormy or tide invaded coast. Tn 
the former case their owners in shallow water would have been 
*On like Mechanical Conditions as producing like Morphological Effects. 
Amer. Nat., 1878, p. 157. 
