354 PEOF. T. W. BRID&B ON THE MOEPHOLOGT OF THE SKULL 



circumorbital ossicles, the skull-bones of Ceratodus correspond with those of Lepido- 

 siren, although differing somewhat from them in their regional extension and their 

 precise relations to the chondrocranium. 



The dermal ethmoid is a relatively larger bone than in other Dipnoi, and extends so 

 far posteriorly as to project beyond the mesethmoid region, and by its sntural union 

 with the " scleroparietal " (Giinther) widely separates the two ectethmoids. The bone 

 identified by me as the equivalent ofthe Amphibian dermal ectethmoid is a much larger 

 and more important element than in Lejndosiren, and has already been sufficiently 

 described (p. 331). The most noticeable feature about it is the downward growth and 

 lateral thickening of its preorbital portion, so that a conspicuous bony antorbital protu- 

 berance separates the nasal from the orbital region of the skull. The mesial area on the 

 dorsal surface of the skull, between the two dermal ectethmoids, which in Lepidosiren 

 is occupied by an extension of the temporal fasciae, is filled up in Ceratodus by a thin, 

 even partially transparent lamina of bone — the " scleroparietal " of Giinther, or the 

 postero-median bone of Huxley. The squamosal bone consists of two distinct but 

 nevertheless continuous portions, the representatives of Huxley's " preopercular " 

 and " proper squamosal " divisions. The first of these overlies the suspensorial 

 cartilage and is a relatively narrow bone, evidently the equivalent of the entire 

 squamosal of Lepidosiren. The second portion is an extension of the dorsal extremity 

 of the former in the shape of an expanded, fan-like, but thin plate of bone, 

 situated externally to the jaw-muscles, and articulating superiorly by an extensive 

 suture with the outer margin of the postorbital portion of the dermal ectethmoid 

 and with one of the circumorbital ossicles. The hinder portions of the dermal 

 ectethmoids, the " scleroparietal," and the dorsal portions of the squamosals all 

 combine to form the eminently characteristic supracranial roof. As regards the mode 

 of development of these various factors, there is to my mind little doubt that they are 

 ossifications of the temporal fasciae. The parasphenoid is a purely ichthyic bone, and 

 in no way contributes to the formation of the lateral walls of the cranial cavity. It is a 

 relatively much larger bone than in other Dipnoi, extending so far anteriorly as slightly 

 to overlap the palatine symphysis and entirely hide from view the extensive area of 

 basicranial cartilage which is freely exposed behind the symphysis in Lepidosiren, 

 while laterally it slightly overlaps the inner smfaces of the two palato-pterygoid bones. 

 Posteriorly the bone extends for a relatively greater distance beneath the notochord 

 than in other Dipnoi, reaching to about the origin of the third pair of ribs. 



The palato-pterygoid bones are less massive structures, and their symphysial 

 portions, although disposed much as in Lepidosiren and overlapped by the nasal sacs, 

 are but moderately thick plates. Owing to the smaller size of the tooth-plates, there 

 are no conspicuous lateral projections from the palatine bones for their support, and 

 there is no obvious representative of the " processus antorbitalis " of Lepidosiren. A 

 feebly-developed ascending process is present, and articulates with a descending plate 

 derived from the preorbital portion of the dermal ectethmoid, the two processes being 



