368 PEOF. T. W. BEIDGE ON THE MOEPHOLOG/Y OF THE SKULL 



i. The existence of endochondrial ossifications. — There is not wanting evidence that 

 in some fossil forms the cartilaginous elements of the skull must have been replaced 

 by bone to an extent which has no parallel in existing types. Thus, Traquair, who 

 was the first to demonstrate the autostylic character of the skull in the Dipteridse and 

 thereby conclusively prove the accuracy of the earlier suggestion of Giinther [i i] as to 

 the relationship of this family to the Dipnoi, in his important paper on the skull of 

 Bipterus [37], remarks "that the chondrocranium was very much more extensively 

 ossified than that of Ceratodus ; in fact, its side-walls were entirely occupied by 

 bone apparently as far as the interorbital region." " The bony matter surrounding 

 the foramen magnum may be held to represent the exoccipitals, and in front of 

 it on each side the walls of the otic region are distinctly ossified — though, from 

 the abraded condition of this part in all the skulls, it is hardly possible to trace 

 any sutural lines marking off distinct osseous elements" (I. c. p. 5). There is also said 

 to be a distinct quadrate provided with a facet for articulation with the mandible. 

 " In fact, we have here before us, as nearly as possible, a counterpart in bone of the 

 suspensorial cartilage of Ceratodus''' (I. c. p. 6). Further, in the same Dipnoid there 

 is an ossified articular element in the lower jaw (Traquair, I. c), in addition to the usual 

 angular and splenial bones. 



There is no satisfactory evidence of the existence of endochondrial bones in any other 

 extinct Dipnoi, with the possible exception of the bone termed " Gehdrkapsel " in 

 Ctenodus (Fritsch, I. c), and doubtfully regarded as a periotic ossification. In the 

 latter Dipnoid, according to Fritsch, there is an ossified cerato-hyal and a basihyal 

 and, in addition, certain bones which he conjectures may represent the cerato-branchial 

 elements of some of the branchial arches. 



ii. The presence of additional dermal bones in connection with the upper and lower 

 jaws. — Of these we have examples in the median dentary at the mandibular (splenial) 

 symphysis of Bipterus (Traquair, 7. c), and the premaxillse and maxilla? which support 

 the pointed conical " marginal " teeth of Phaneropleuron (Traquair, 38), and, in addition, 

 the several bones which, in the lower jaw of the same Dipnoid, carry similar teeth. 

 The presence of " marginal teeth " and supporting dermal bones in this genus must, 

 however, be regarded as at least open to doubt, although usually cited as one of the 

 distinctive characters of the Phaneropleuridee. 



In a paper subsequent to the one quoted above, Traquair [39] remarks of Phanero- 

 pleuron andersoni : — " As for the conical teeth described by Huxley, I have satisfied 

 myself that they are merely tne outer denticles of ctenodont plates. Whiteaves's 

 statement that in Scaumenacia curta ' both the upper and under jaws are armed with 

 smooth, conical, and somewhat compressed teeth,' I have never been able to confirm — 

 at least, if marginal teeth are hereby meant " (I. c. p. 264). 



iii. The presence of jugal plates. — Bipterus has two pairs of such plates — an anterior 



