GANOCEPHALA 179 



would be equal to the performance of the required amount of 

 respiration ; but the retention of such traces of the embryonal 

 water-breathing system in the adult leads to the inference 

 that the animal must have affected a watery medium of exist- 

 ence for as great a proportion of its time as is observed to be 

 the case in the existing perennibranchiate reptiles ; in which, 

 notwithstanding the degree of development of the lungs, the 

 respiratory function seems to be mainly performed by the 

 gills. 



The additional marks of affinity to fishes which the Archer 

 gosaurus presents in its persistent notochord, cartilaginous 

 basi-occipital, dermal ossifications on the head, and minute 

 body-scales (fig. 65, scales), remove it further from the saurian 

 reptiles, and exhibit it more strongly in the light of an osculant 

 form between the Batrachians and the Ganoids. 



The under surface of the body between the head and trunk 

 is defended by broad bony plates, three in number. One is 

 median and symmetrical, of an elongate lozenge shape, with 

 the angles rounded off ; slightly convex externally, a little 

 produced along the middle of the anterior half into something 

 like a low quasi-\&e\. The outer surface is sculptured by 

 radiating furrows, except at so much of the marginal part as 

 is overlapped by the lateral pieces, and by the scapular arch. 

 The lateral throat-plates are attached to the anterior half of 

 the sides of the median one, are shaped like beetles' elytra, and 

 converge forwards. Their centre of ossification is towards 

 their outer and back part, from which the external ridges and 

 grooves radiate towards the inner border. 



Von Meyer* compares these dermal shields to the ento- 

 and epi-sternal elements of the plastron of Chelonia ; their 

 truer homology seems to the writer to be with the median and 

 lateral large throat-plates or scales of Mcgcdiclithys and Sndis 



* " Die kehlbvust platten konnte man der unpaarigen Platte und dem ersten 

 Platten-paav im Bauchpanzer der SchildkriJter verglcichen." {Op. tit., p. 100.) 



