378 HOWARD AYERS 
not to mention other groups. The acquisition of biting jaws 
was an important event in the evolution of vertebrates, for it 
marked a great advance in the mechanics of food-gathering and 
of defence. What advance, if any, Bdellostoma has made toward 
the acquisition of a maxillary mechanism is not particularly 
clear. Allis is the only investigator who has sought to identify 
the maxillary element by comparative means, taking into con- 
sideration skeleton, nerves, and muscles. His conclusion that 
the cartilage of the fourth tentacle represents the cartilaginous 
Fig. 28 Face view of 37-mm. embryo before dissection was made for figures 
26 and 27. Note the small buccal aperture and the relations of tentacles to both 
nasal and buccal openings. 
precursor of the osseus maxilla of Ganoids and Teleosts and that 
the lateral labial cartilage is in fact the cartilaginous palatine 
element does not appear to me to be well founded. Iam not yet 
willing to say that it is impossible, but it is very improbable in 
my judgment. This much is certain, the fourth tentacle is used 
by Bdellostoma as an antagonist to the jaw as already described, 
in that it is at times pressed against the dental surface of the 
dentigerous mandible. In the event the fourth tentacle proves 
to be the nascent maxilla, we would have the whole mandibulo- 
maxillary apparatus of vertebrates developed out of the Amphi- 
