1 98 PALAEONTOLOGY. 



are the pair of bones answering to the "mastoids" (8). They 

 give attachment externally and below to the tympanic (28), 

 and to a subsidiary bony plate (supra-squamosal, s) holding 

 the position of that development of the mastoid and squa- 

 mosal, which roofs over the temporal fossa in the Chelonia. 

 The frontal bones (n), divided by a mid-suture, like the parie- 

 tals, increase in length, and are continued far in advance of 

 the orbits. The bone which occupies the position of the post- 

 frontal in Chelonia is ossified from two centres, one (12) articu- 

 lating with the mastoid ( 8 ), the other, post-orbital, 0, with the 

 supra-squamosal. The post-frontal extends forward above the 

 orbit to meet the pre-frontal, separating the frontal (n) from 

 the orbit, as in the sturgeon (Acvpenser), Polypterus, and Lepi- 

 closteus, and also in some Chelones. The pre-frontal extends 

 far forward, terminating in a point between the nasal (15) and 

 lacrymal. The nasals (15), divided by the median suture, 

 extend to the external nostrils, their prolongation varying 

 with the species and age of the individual. 



Thus far the ossification of the superficies of the skull of 

 Archegosaurus closely conforms to that of the salamandroid 

 ganoid fishes above cited ; and the homologous bones are 

 determinable without doubt. The lacrymal bone obviously 

 answers to the front large suborbital scale-bone in fishes ; its 

 large size and forward extension in Archegosaurus is a mark 

 of that affinity. 



The upper jaw consists of pre-maxillary (22), maxillary 

 (21), and palatine bones. The pre-maxillaries are divided by 

 a median suture, as in Lepidosteus and Crocodilus, and are 

 short bones, the breadth exceeding the length in A. latirostris, 

 and also in the young of A. Decheni ; but in the old animal 

 opposite proportions prevail. The maxillary (21) extends from 

 the pre-maxillary to beneath and beyond the orbit. The 

 palatine is a long narrow bone, rather expanded at both ex- 

 tremities ; it forms anteriorly the hinder border of the choanal 



