FOSSILS AND PEDIGREES 347 



skill and patience displayed by Dr. Stensio, who has made a 

 special study of these remains, it is possible to see that the 

 arrangement of the nervous and vascular systems bears some 

 marked resemblance to that of existing Cyclostomes. Dr. Stensio 

 believes that the very large nerves which extend upwards and 

 outwards from the hinder part of the brain supplied electric 

 organs, probably embedded in the head-shield. On the lower 

 surface of the shield is a series of separate gill-openings on each 

 side, and from the evidence supplied by markings on the inner 

 side of the shield it appears certain that the gills themselves 

 were contained in pouches. An important feature of these 

 structures is the position of the first pair of pouches, which is 

 in advance of that occupied by the jaws in other vertebrates. 

 The significance of this becomes clear when it is considered 

 that in the Selachians and Bony Fishes the modification of one 

 pair of gill-arches into biting jaws has entailed the disappearance 

 of the gills in front of them, and it follows, therefore, that the 

 Cephalaspids cannot have possessed true jaws, nor can they 

 have been descended from animals with such jaws. As in the 

 Anaspids, there is a single nostril in the middle of the upper 

 surface of the head-shield. In the auditory region there are 

 only two semicircular canals, as in the existing Lampreys, the 

 horizontal canal of other fishes being undeveloped. A study 

 of the development of the Lamprey provides still more evidence 

 of its relationship to these Palaeozoic forms, for the plate of 

 so-called mucous cartilage which arises in the head region of the 

 larva corresponds remarkably closely, both in form and position, 

 with the head-shield of Cephalaspis — a very striking example of 

 recapitulation! The Cephalaspids were doubtless sluggish 

 creatures that lived on the bottom, and in their shape and 

 manner of life bear much the same relationship to the Anaspids 

 as do the Rays to the Sharks. Towards the middle of the 

 Devonian they gradually decreased in abundance, became 

 comparatively rare during the latter half of that period, and 

 finally became extinct before the beginning of the Carboni- 

 ferous. The causes which led to their disappearance from the 

 face of the earth must remain a matter for speculation, but 

 several similar cases occur in other groups of fishes, where 

 elaborate, clumsily built, over-armoured creatures have quite 

 suddenly died out, giving place to less specialised and less 

 protected, but more active forms. 



The last sub-class of Palaeozoic Marsipobranchs, the Hetero- 

 straci, likewise occur in Silurian and Devonian strata. They have 



