852 The American Naturalist. [Octobef, 
CRANIATA. 
I. No lower jaw nor pectoral arch. 
Internal skeleton not ossified ; Agnatha. 
II. Lower jaw and pectoral arch present. 
a. Basicranial axis not ossified ; vertebral column con- 
sisting chiefly of intercentra. 
Limbs represented by many-radiate fins, which are 
also present on the median lines of the body ; 
a coracoid bone ; heart with two chambers ; no 
internal nares ; Pisces. 
Limbs consisting of one basal element, two propodials, 
and metapodials and digits ; no median fins ; no 
opercular bones ; a coracoid ; heart with three 
chambers ; two occipital condyles ; internal 
nares ; Batrachia. 
aa. Basicranial axis ossified ; vertebral column consist- 
ing chiefly of centra. An amnion and allantois. 
Limbs as in Batrachia ; one occipital condyle ; a sus- 
pensorium of the lower jaw ; mandible seg- 
mented ; heart with three^ chambers ; Monocondylia. 
Limbs as in Batrachia ; two occipital condyles ; no 
suspensorium of the lower jaw ; mandible not 
segmented ; heart with four chambers ; Mammalia. 
AGNATHA. 
The known members of the class Agnatha are a very small 
representation of those that once existed ; and they present a 
great variety of character, having little affinity with each other. 
Two sub-classes are most distinctly indicated. 
An osseous dermal skeleton with lateral limb-like 
appendages ; Ostracodermi. 
No osseous skeleton, nor lateral limb-like append- 
^^^^ ' Marsipobranchn. 
OSTRACODERMI. 
The orders of this division are two. 
Orbits well separated ; no nostrils ; Arrhina. 
