THE EVIDENCE OF THE SKELETON 125 
structures in the fossils of the Silurian age either among the verte- 
brate or invertebrate remains. Fortunately for this investigation 
there are still living on the earth two representatives of that age; on 
the invertebrate side Limulus, and on the vertebrate side Ammoccetes. 
The Elasmobranchs represent the most primitive of the gnatho- 
stomatous vertebrates. Below them come the Agnatha, known as the 
cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon) 
and the hag-fishes (Myxine). 
The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial 
skeleton composed of a cartilaginous unsegmented cranium, with the 
basal trabeculze and parachordals and a series of branchial and visceral 
cartilaginous bars forming the so-called branchial basket-work; to 
these must be added auditory and nasal capsules. In contradis- 
tinction to this elaborate cranio-facial skeleton, the spinal vertebral 
Fic. 52,—SKELETON OF PreTromyzon. (From Parker.) 
na., nasal capsule; az., auditory capsule; nc., notochord. 
skeleton is represented only by segmentally arranged small pieces of 
cartilage formed in the connective tissue dissepiments between 
segmented sheets of body-muscles (myotomes). 
But Petromyzon is derived from Ammoccetes by a remarkable 
process of transformation, and a most important part of that trans- 
formation is the formation of new cartilaginous structures. Thus we 
see that in Ammoccetes there is no sign of a cartilaginous vertebral 
column ; at transformation the rudimentary vertebra of Petromyzon 
are formed. In Ammoccetes the brain-case is a simple fibrous mem- 
branous covering; at transformation this becomes cartilaginous. In 
Ammoccetes there are no cartilaginous structures corresponding to 
the sub-ocular arches; these are all formed at transformation. It 
follows, that we can trace back the bony skeleton of the vertebrate 
head to the skeleton of Ammoccetes, and we may therefore conclude 
