THE EVIDENCE OF THE SKELETON [45 
of the muscular attachments cross between the two longitudinal 
trabecule, and so form the transverse trabecule. 
I entirely agree with Schimkéwitsch that the nests of cartilage- 
cells are much more extensive in, and indeed nearly entirely 
confined to, these two lateral trabecule in the entosternite of 
Hypoctonus. Ray Lankester describes in the entosternite of Mygale 
peculiar cell-nests strongly resembling those of Hypoctonus, and he 
also states that they are confined to the lateral portions of the 
entosternite. 
From this evidence it is easy to see that that portion of the basi- 
cranial skeleton known as the trabecule may have originated from 
the formation of cartilage in the plastron or entosternite of a pale- 
ostracan animal. Such an hypothesis immediately suggests valuable 
clues as to the origin of the cranium and of the rest of the basi- 
cranial skeleton—the parachordals and the auditory capsules. The 
former would naturally be a dorsal extension of the more membranous 
portion of the plastron, in which, equally naturally, cartilaginous tissue 
would subsequently develop ; and the reason why it is impossible to 
reduce the cranium into a series of segments would be self-evident, 
for even though, as Schimkéwitsch thinks, the plastron may have 
been originally segmented, it has long lost all sign of segmentation. 
The latter would be derived from a second entosternite of the same 
nature as the plastron, but especially connected with the auditory 
apparatus of the invertebrate ancestor. The following out of these 
two clues will be the subject of a future chapter. 
In our search, then, for a clue to the origin of the skeletal tissues 
of the vertebrate we see again that we are led directly to the paleos- 
tracan stock on the invertebrate side and to the Cyclostomata on that 
of the vertebrate; for in Limulus, the only living representative of 
the Paleostraca, and in Limulus alone, we find a skeleton marvel- 
lously similar to the earliest vertebrate skeleton—that found in 
Ammoccetes. Later on I shall give reasons for the belief that the 
earliest fishes so far found, the Cephalaspide, etc., were built up on 
the same plan as Ammoccetes, so that, in my opinion, in Limulus 
and in Ammoccetes we actually possess living examples allied to 
the ancient fauna of the Silurian times. 
