4 
POPULAR SCIENCE REVIEW. 
neural and haemal processes of firm opaque cartilage are also 
present. 
This absence of a solid vertebral column and bony frame- 
work does not, however, necessarily imply a poverty of lime in 
such animals, for many fishes in which the notochord is unossified 
have well-developed neural and haemal processes, or have theii 
heads and bodies more or less encased in stout bony plates* 
forming a powerful defensive armature. (See PL II., Fig. 1, 
Coccosteus.) Indeed, bony matter may be variously disposed in 
the bodies of vertebrate animals. 
The 44 Trunk-fish ” ( Ostracion bicav,- 
dalis , PI. II., Fig. 3), the Tortoise, and 
the Armadillo, are instances of its accu- 
mulation upon or near the surface of the 
body, constituting the 44 dermo-skeleton 
the deep-seated bones in relation to the 
bony-framework forming the 44 neuro- 
skeleton,” being also present in these and 
many other instances. (See Woodcut, 
Fig. 2.) 
It is important, however, not to con- 
found this dermo-skeleton in the Verte- 
brata with the exo-skeleton in the Inverte- 
brata. 
The shells and crusts of molluscous 
and crustaceous animals are unvciscular ; 
they grow by the addition of layers to their 
circumference, they may be cast off when too small for the 
growing body, and be reproduced of a larger size. When frac- 
tured, the broken parts may be cemented together with fresh 
shell-substance, but are not unitable by the action of the 
fractured surfaces from within. 
Bone, on the contrary, is a living vascular part, growing by 
internal molecular addition and change, and having the power 
of repairing fracture or other injury (Owen). 
It may seem somewhat needless to recall to mind these 
elementary details ; but in speaking of the earliest known fossil 
Vertebrata, it must be borne in mind that in many instances 
they are represented only by remains of their dermo-skeletons, 
the microscopic structure of which is frequently all that can be 
definitely relied upon to class them with the Vertebrata. 
To the late Sir Roderick Murchison is due the honour of 
having, in company with Mr. Hugh Strickland, first discovered, 
in 1837, two thin 44 bone-beds,” each little more than one inch 
thick, and separated by about fifteen feet of fossiliferous Upper 
Ludlow rock in a cutting of the Gloucester and Ross railroad. 
Alt hough several of the supposed fish-remains from the 
Fig. 2. 
SEGMENT OF NF.URO- AND 
DERMO-SKELETONS OF 
Ostracion bicaudalis 
(after Owen). 
dn , dp, dh. dermal plates. 
n. neurapophyses. 
pi. pleurapophysps. 
