174 PALAEONTOLOGY 



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, holding the position of that 

 development of the mastoid and squamosal, which roofs over 

 the temporal fossa in the Chelonia : it may be termed " supra- 

 squamosal" (the bone between 8 and 27 in fig. G5). The 

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

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

 orbits. The bone (12) which occupies the position of the post- 

 frontal in Chelonia is ossified from two centres, one articulating 

 with the mastoid ( 8 ), the other, which is external to it, with 

 the supra-squamosal. This other bone may be termed the "post- 

 orbital," as proposed by Von Meyer. The post-frontal extends 

 forward above the orbit to meet the pre-frontal, separating 

 the frontal (n) from the orbit, as in the sturgeon {Acipenser}, 

 Polypterus, and Lepidosteus, and also in some Chclones. 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 prolonga- 

 tion 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. In A. Decheni each pre-maxillary 

 contains eight teeth; in A. latirostris not less than eleven. 



