170 Prof. H. G. Seeley on the 



inclined external surfaces, which are formed by the post- 

 frontal bones, from the parietal bones upon which they rest. 

 Underneath these areas the skull is excavated in the usual 

 way. 



The parietal bones are three in number. At their junction 

 the parietal foramen is placed ; it is oblong, keyhole-shaped, 

 rather wider behind than in front, yV inch long, f'^ inch wide 

 behind, and more than half that width in front ; its transverse 

 hinder border is j^ inch from the posterior occipital ridge. 

 The two parietal bones, which meet behind the parietal 

 foramen in a sinuous median suture, form a slight inflation 

 where the hinder end of the suture meets the interparietal 

 bone. The flat parietals extend transversely outward to the 

 margin of the inclined concave temporal vacuity formed by 

 the postfrontal bone. Anteriorly the two parietal bones 

 diverge, and a somewhat heart-shaped median preparietal 

 bone is contained between the long narrow anterior processes 

 of the parietals. External to those processes the frontal 

 bones are prolonged backward upon the parietal bones, so as 

 to divide their anterior border into a longer internal process 

 and a shorter outer process. The single median bone (pre- 

 parietal) is lyV iiicli long, though not measuring more than 

 an inch in tlie median line, because its narrow liinder margin 

 is notched out by the parietal foramen. It is yV inch wide in 

 front where widest, and its convex serrated anterior border 

 extends forward somewhat in advance of the narrow inner 

 lateral parallel processes of the parietal bones which flank it. 

 It is usually regarded as a parietal bone ; but it is in the 

 linear position of the interparietal, ethmoid, and intermaxillary 

 bones, as occupying a median position in the skull alter- 

 nately with the paired bones of the brain-case and face, such 

 as the parietal and frontal. In osteology it has been some- 

 times treated as though it were the principal parietal, when 

 the tAYO posterior bones are often described as its posterior diver- 

 gent processes among existing reptiles, when the interparietal 

 bone is not separately ossified. Reasons have been urged for 

 comparing these paired and unpaired ossifications with those 

 found arching over the neural canal in the spinal column in 

 Lnmna and other Elasmobranch fishes, in which there is a 

 similar alternation of paired and unpaired bones, which 

 suggest a certain homology between the cranial and vertebral 

 structures ; and since the single median bone now described 

 has much the same relation to the frontal bones that the 

 interparietal has to the parietal bones, it may be known as 

 the preparietal ; it appears to be a good distinctive feature of 

 these Anomodont skulls. It probably disappears in many 



