348 THE ORIGIN OF VERTEBRATES 
tissues in the head-region of Ammocctes. Fig. 141 represents a 
section through the head near the pineal eye. Most internally is a, 
a section of the membranous cranium, then comes 8, the muco- 
cartilaginous skeleton, then ¢, the laminated layer, and finally ¢, the 
external cuticle. If in Ammoccetes we possess an epitome of the 
history of the vertebrate, how would these layers be represented in 
the past ages, supposing they 
could be fossilized ? 
The most internal layer «, by 
the formation of cartilage and 
then bone, represents the great 
mass of vertebrate fossils; the 
next layer 5, by a process of 
calcification, as previously argued, 
represents the head-shield of the 
Osteostracan fishes; while the 
cuticular layer d, no longer thin, 
is the remnant of the Paleo- 
stracan head-carapace. Between 
these two layers, 6 and d, lies the 
laminated layer c. Intermediate 
to the Palzeostracan and the Osteo- 
stracan comes the Heterostracan, 
with its peculiar head-shield—a 
head-shield whose origin is more 
easily conceivable as arising from 
Fic. 141.—SEctTIon oF SKIN AND UNDER- 
LYING TISSUES IN THE HiAD-REGION 
or AMMOC@TES, 
u, cranial wall; 6, muco-cartilage; c, 
laminated layer; d, external cuticular 
layer. 
something of the nature of the 
laminated layer than from any 
other structure represented. in 
Ammoceetes. 
My present suggestion, then, 
is this: the transition from the 
skeletal. covering of the Paleostracan to that of the highest verte- 
brates was brought about by the calcification of successive layers 
from without inwards, all of which still remain in Ammoccetes and 
show how the external chitinous covering of the arthropod was 
gradually replaced by the deep-lying internal bony cranium of the 
higher vertebrates. 
In Ammoceetes the layer which represents the covering of the 
