238 DIPNOI 



(Fig. 206). But the hyomandibular takes no share in the support 

 of the jaws. It disappears, indeed, entirely in the Dipneumones, 

 where the ceratohyals alone remain, and, as Huxley showed [230] 

 (Ridewood [358], Sewertzoff [408]), is represented in Ceratodus by a 

 minute vestigial cartilage, overlying the hyomandibular branch of 

 the seventh nerve (Fig. 206). Of the structure of the hyoid and 

 branchial arches in the fossil forms we know practically nothing, but 

 there is no reason to think that it differed essentially from that of 

 modern Dipnoi. Traquair has shown that Dipterus was autostylic 

 [447]. The branchial arches in Ceratodus are fairly well developed 

 with epibranchial elements, and even some pharyngobranchials ; 

 but in the Dipneumones they are reduced to mere unsegmented 

 cartilaginous rods. On the other hand, a small cartilage in front 

 of the first branchial slit led some observers (Wiedersheim [489], 

 Bridge [56a]) to believe that there are six branchial arches in the 

 Dipneumones, one gill-cleft having disappeared behind the hyoid 

 arch. But the distribution of the nerves, blood-vessels, etc., 

 does not support this view, and the cartilage rod appears to be 

 a secondary development from the base of the gill-rakers (K. 

 Furbringer [141«]). 



Among the chief characters of the dermal bones of the skull 

 which distinguish modern Dipnoi from the more primitive early 

 Osteichthyes, we may mention the sinking of the bones beneath 

 the surface, leaving the sensory canals in the skin, the overgrowth 

 of these bones by scales, and their reduction in number ; the pre- 

 ponderance of large median elements, the loss of the nasals, 

 premaxillae, and maxillae, and the great reduction or entire dis- 

 appearance of the dentary ; the correlated absence of teeth on the 

 margin of the mouth ; the presence of a pair of strong pterygo- 

 palatine bones, bearing large compound palatine teeth ; the 

 development of corresponding large teeth below on the splenials ; 

 the absence of the ventral and lateral gular series, the small size of 

 the opercular and subopercular, and the absence of a pineal foramen. 

 So far have these Dipnoi departed from the normal type, that the 

 homology of the cranial bones cannot yet be determined with 

 certainty. 



Passing backwards to the Devonian forms, we find skulls more 

 nearly resembling those of the primitive Teleostomes. In the 

 Phaneropleuridae (Fig. 208) are large paired frontals and parietals, 

 meeting in the middle line, followed behind by a median occipital. 

 The supraorbital sensory canal is carried by a row of plates, the 

 prefrontal, supraorbitals, and postfrontal. It is continued back- 

 wards through two posterior bones, probably representing the 

 squamosal (pterotic) and supratemporal. A chain of bones, enclos- 

 ing the infraorbital canal, passes below the orbit from the post- 



