DEVELOPJIENT OF THE SKULL IN THE UEODELES. 173 



Over the foramen magnum they unite to form a rounded process, just as the azygous 

 l^arasphenoid (PI. XXXVII. fig. 1, pa.s) forms a rounded process under this archway, 

 but much further back. On each side, over the broad free end of the hourglass-shaped 

 occipital cincture, each parietal sends a bevelled, ovoid, dentate plate ; in front of this 

 plate each bone is higher and thicker, and at its anterior notched edge articulates 

 with the squamosal (sq). Again, in the temporal region each bone is developed into 

 a short spur, in front of which, in the postorbital region, the bone bends inwards, and 

 then stretches gently outwards to its pointed fore end, where it lies on the ethmoid, and 

 binds the outer face of the prefrontal (^j/) ; the sagittal suture is formed by the 

 meeting of the raised inner edges of the two bones. 



The frontals (PI. XXXVl. fig. 1,/) reverse the form of the parietals; they are 

 sharp wedges behind and are dilated in front, where they diverge abruptly to let in the 

 nasals (•«). These bones reach backwards halfway between the foramen opticum and 

 the f. ovale (ir, v) ; in front the diverged part binds against the top of the maxillary 

 (ma') and the inner edge of the prefrontal (j).f), whilst the nasals (n) overlie the notch 

 formed by their divergence. 



The frontal suture has large irregular teeth ; and the edges are sharp where the two 

 bones are interlocked (fig. 2). 



The nasals (n) together form an oxhead-shaped plate, the cheeks of which articulate 

 with the top of the maxillaries, whilst the horns run in between the nasal processes of 

 the premaxillaries and the nasal cartilage surrounding the outer nostrils (na, e. n). 



Their narrow hinder end rests by a squamous suture upon the notched frontals ; their 

 median suture has strong teeth ; their surface is hollow towards the middle, subsca- 

 brous, and pitted. 



There are only two pairs of supero-lateral bones, viz. the prefrontals and squamosals. 

 The former (^j./) are irregular conchoidal splints, lying outside the fore half of the 

 frontals, and articulating also by their front edge with the ascending plate of the 

 maxillaries. They are pinched in at the anteorbital region, and only half conceal the 

 nasal roof («a) ; their outer and fore margin is notched; and they overlap the frontals 

 by a squamous suture. They are half the size of the frontals. 



The squamosals [sq) are strong, oblong, carinate bones ; they articulate above with 

 the parietals by a rounded head with raised borders, are crested in a rugged manner 

 along their outer surfiice, and the crest crosses over from the hinder part of the upper 

 ribbed edge to the fore edge below. The hind edge is rounded, and the front edge 

 rather straight, so that they are semielliptical below ; they bind strongly on the half 

 bony suspensorium. Here we see the economy used in this group as to bony plates ; 

 for this squamosal answers to the proper squamosal (temporal) of most Ganoids and 

 of the Siluroid Teleosteans, and also to the preopercular. 



There are only two bones in the marginal arc ; for each premaxillary has become 

 ankylosed to the maxillary [px, mx) of the same side; but the former are only one 



2s2 



