THE EVIDENCE OF THE SKELETON 145 



of the muscular attachments cross between the two longitudinal 

 trabecular, and so form the transverse trabecule. 



I entirely agree with Schimkewitsch that the nests of cartilage- 

 cells are much more extensive in, and indeed nearly entirely 

 confined to, these two lateral trabecular in the entostemite of 

 Hypoctonus. Kay Lankester describes in the entostemite of Mygale 

 peculiar cell-nests strongly resembling those of Hypoctonus, and he 

 also states that they are confined to the lateral portions of the 

 entostemite. 



From this evidence it is easy to see that that portion of the basi- 

 cranial skeleton known as the trabecular may have originated from 

 the formation of cartilage in the plastron or entostemite of a pake- 

 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 Schimkewitsch thinks, the plastron may have 

 been originally segmented, it has long lost all sign of segmentation. 

 The latter would be derived from a second entostemite 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 palaros- 

 tracan stock on the invertebrate side and to the Cyclostomata on that 

 of the vertebrate ; for in Limulus, the only living representative of 

 the Palaeostraca, and in Limulus alone, we find a skeleton marvel- 

 lously similar to the earliest vertebrate skeleton — that found in 

 Ammocoetes. Later on I shall give reasons for the belief that the 

 earliest fishes so far found, the Cephalaspidae, etc., were built up on 

 the same plan as Ammocoetes, so that, in my opinion, in Limulus 

 and in Ammocoetes we actually possess living examples allied to 

 the ancient fauna of the Silurian times. 



