4 THE AMERICAN NATURALIST. [Vor. XXXIV. 
6. Cervicals 3-9. The ventral intermediate pieces (7.3 to 2. in 
our figures) are spoken of as *intercentra," also as **hypapophyses." 
I. RaAcHITOMOUS PROTOTYPES. 
For the origin of these structures we must' naturally turn 
back to extinct forms, and we find two rachitomous prototypes 
— one composed of six pieces, one of five. 
Among the primitive Amphibia (Stegocephalia) we find types 
in which every vertebra is composed of six pieces, as figured by 
—--ntercentrum. 
|o 
Cond A it : DX. Ww t ç. 
Fic. 3. — Discosaur Permian of S 
ed from the Zz/? side, after Credner. Ne urap — xe arch, 2 pieces; site 
centra, lateral, 2 pieces; intercentra, median ventral, 1 pie 
Fritsch, Fig. 2 (Chelydosaurus vranit). It is possible, therefore, 
that the “proatlas” may represent, not a separate vertebra, but part 
of the atlas of a persistent rachitomous type.! This is improbable. 
1 According to this hypothesis, for which at present little can be said, the 
homologies would be as follows: 
Atlas Components. Secondary. 
Primitive neurapophysis = proatlas. 
Primitive pleurocentrum = neural arch. 
Primitive intercentrum = base of atlas ring. 
Primitive hypocentrum pleurale = odontoid process. 
The demonstration of a vestigial, anterior, cervical nerve belonging to the 
— “ proatlas” vertebra would be fatal to the above hypothesis. 
