400 HOWARD AYERS 
elements of Bdellostoma being fused together in a cartilaginous 
unit. 
A maxillary region is marked out by the presence of teeth and is, 
however, limited behind by the articulation of themandible with the 
base of the skull. The hyoidean base plate is still present in the 
form of a large support plate below the mandible, and we are thus 
supplied with an instructive intermediate stage between the 
Myxinoids and the sharks in which the Amphioxine jaw appa- 
ratus is still present in its modified Myxinoid form with a well- 
defined mandible underlaid by the old time Myxinoid hyoidean 
support. The parts of the distal section of the ancestral jaw 
apparatus are not fused together nor are they fused with the 
cranium. Butanew relation of parts has appeared—the mandible 
is articulated to the base of the cranium, being tied to its ful- 
crum by strong tendons. From this time on the distal section 
of the jaw apparatus fades away, the hyoidean apparatus under- 
goes reduction, the second mandible disappears and the newly 
acquired maxillomandibular articulation becomes the seat of 
stresses which call forth many different mechanical devices in 
the effort to improve the biting jaws and to adapt them to special 
conditions of food gathering and defense which are constantly 
arising in the efforts of animals to adjust themselves to their 
environment. The conditions of life become complicated and 
survival more difficult and the differentiation of the biting Jaws 
is an expression of this struggle. We thus see that Bdellostoma 
has not lost definitive maxillary cartilages because they were 
not developed in vertebrates until the Myxinoid stage was passed. 
However, the fundamental cartilages of the head framework 
with which the maxillary mechanism is associated and to which 
they are more and more mechanically united as we ascend the 
vertebrate scale are well developed. The pulls and stresses 
set up in the skeletonous tissues by the movements of the head 
parts have caused the appearance of cartilage and chondral 
tissues in those localities most needing support and those which 
form the foci of the mechanical stresses due to the pull of the 
contractile tissues, until in Bdellostoma the basic cranial carti- 
lages found in the vertebrate stock above the Myxinoids in great 
and increasing variety and flux, have been built up into a head 
