RELATIONSHIP OF AMMOCGTES TO OSTRACODERMS 331 
were preserved. Of these, some would indicate the position of blood- 
vessels, such, for instance, as of the external carotid which traverses 
this structure; but the largest and most internal spaces, resembling 
Rohon’s medullary spaces, would represent muscles, being filled up 
with bundles of the upper lip-muscles. 
THE Muco-CaRrTILAGINOUS HEAD-SHIELD oF AMMOCCETES. 
The resemblance between the structure of the head-shield of 
Thyestes and the muco-cartilage of Ammoccetes, is most valuable, 
for muco-cartilage is unique, occurs in no other vertebrate, and every 
trace of it vanishes at transformation; it is essentially a character- 
istic of the larval form, and must, therefore, in accordance with all 
that has gone before, be the remnant of an ancestral skeletal tissue. 
The whole story deduced from the study of Ammoccetes would be 
incomplete without some idea of the meaning of this tissue. So 
also, as already mentioned, the skeleton of Ammoccetes is incomplete 
without taking this tissue into account. It is confined entirely to 
the head-region ; no trace of it exists posteriorly to the branchial 
basket-work. It consists essentially of dorsal and ventral head- 
shields, connected together by the tentacular, metastomal, and thyroid 
bars, as already described. The ventral shield forms the muco-carti- 
laginous plate of the lower lip and the plate over the thyroid gland, 
so that the skeleton ventrally is represented. by Fig. 118, B, which 
shows how the cartilaginous bars of the branchial basket-work are 
separated from each other by this thyroid plate. At transformation, 
with the disappearance of this muco-cartilaginous plate, the bars 
come together in the middle line, as in the more posterior portion 
of the branchial basket-work. 
The dorsal head-shield of muco-cartilage covers over the upper 
lip, sends a median prolongation over the median pineal eyes and 
a lateral prolongation on each side as far as the auditory capsules, 
giving the shape of the head-shield of muco-cartilage, as in Fig. 
118, C. 
Not only then is the structure of the head-shield of a Cephalaspid 
remarkably like the muco-cartilage of Ammoccetes, but also: its 
general distribution strangely resembles that of the Ammoccetes 
muco-cartilage, 
Now, these head-shields in the Cephalaspide and Tremataspide 
