ORIGIN OF JAW APPARATUS 399 
short of preserving the Myxinoid stage of theprimitive jaw appa- 
ratus. They present us with a stage of reduction lying between 
Callorhynchus and the Elasmobranchs, and the parts in Chimaera 
are important for that reason. 
The series of facts presented shows the genetic relationship 
of the skeleton of the jaw apparatus step by step from Amphioxus 
to Ammocoetes to Bdellostoma and to the Chimaeroids, which 
are accepted Gnathostomes. It follows that the associated 
structures, such as muscles, nerves and blood vessels are likewise 
genetically related. As the nerves are dominantly associated 
with the jaw apparatus, an examination of them should disclose 
some indication of this genetic relationship, as after the skeleton 
they are usually less changed during the course of phylogenetic 
development than either the muscles or the vascular supply. 
The results of such an examination of the nerves of Bdellostoma 
have been presented in the foregoing pages. I believe that the 
two conclusions given at the beginning of this contribution are 
therefore fully supported by the evidence submitted. 
It is also evident that the lower jaw or mandible is the first 
part of the biting Jaws to be established. The maxillary struc- 
tures arose in response to the stress and pressures developed 
as the Bdellostomid mandible (which on first sight appears to 
operate largely in a horizontal direction, but which develops its 
main stresses while in a vertical position outside the mouth) 
acquired relations such that these vertical stresses were developed 
with the jaw inside the mouth. With the jaw in action, the 
hyoidean apparatus of Bdellostoma takes up the vertical stresses 
developed in the movements of the mandible, through its muscu- 
lar and tendinous attachments to the skull above it, especially 
the cartilaginous frame which includes the quadrate, palatine, 
and other elements of the skulls of higher forms. But the hyoid- 
ean stresses have not brought forth a definite maxillary anvil to 
take the pressures of the mandibular hammer. They have, 
however, brought forth a chondral condensation in the membranes 
tying the hyoidean base plate to the skull. In the Chimaeroids 
the skull has solidified into cartilage in a large way; only rem- 
nants of the membranous skull remain, all the separate cranial 
