850 The American Naturalist. [October, 
structure, and by the absence from our collections of numerous 
intermediate forms which must have existed. Until our knowl- 
edge is more complete the arrangement, especially of the contents 
of the class Agnatha, must be regarded as largely provisional. 
The ossification of the skeleton of the Vertebrata has developed 
first on the exterior of the head and body, and in the sheath of 
the chorda dorsalis, and has then penetrated inwards. The 
limbs have preceded in time the arches (scapular and pelvic) to 
which they are, in the higher forms, attached. Hence we find in 
such genera as Cephalaspis and Bothriolepis, pectoral limbs with- 
out a scapular arch, but with merely dermal ossifications to which 
they are attached. This is parallel to the general absence of 
pelvic arch in most fishes. The limbs themselves are supposed to 
be radial ossifications in primitive longitudinal folds of the body 
integument, some of which remain in large part, as the dorsal 
fin of various fishes ; while more frequently but few of the radii 
remain, as in the limbs of most Vertebrata. 
The branch Vertebrata is divided into the following super- 
No skull nor skeleton ; notochord short, anterior, 
temporary ; nervous center a longitudinal cord ; Hemichorda. 
No skull nor skeleton ; notochord caudal only ; 
nervous center a ganglion ; Urochorda. 
No skull ; notochord extending throughout the 
body, included in a membranous sheath, as is 
the cord-like nervous axis above it ; Cephalochorda. 
A cartilaginous or bony skull and skeleton, which 
extends throughout the body ; central nerv- 
ous system a longitudinal cord terminating 
in a brain within the skull ; Craniaia. 
HEMICHORDA. 
There is but one class of Hemichorda. 
Not metameric ; no mantel ; respiratory fissures 
on each side of the pharynx ; alimentary 
canal with openings at opposite extremities 
°^ bo^y ' Enteromeusta. 
