ADAMS, PHYLOGENY OF THE JAW MUSCLES 



81 



This small cartilage, then, represents the hyomandibular and the sym- 

 plectic of the teleosts. It is in the correct position for these bones and 

 there is little doubt of the homology. With its reduction there has been 

 quite a change in the opercular region, as the opercular bones are reduced 

 and changed together with the preopercular. Giinther thought that a 

 small bit of cartilage on the anterior edge of the opercular was the vestige 

 of the preoperculum. Goodrich (1909, pp. 237-238) says: 



It is in the connection of the skull with the visceral arches that the dipnoi 

 have diverged most conspicuously from the other fishes. The modern genera 

 are completely autostylic. The pterygo-quadrate bar is firmly fused to the 

 cranium in front and behind. The spiracle disappears and the hyoid arch is 

 well developed, with a medial basihyal, paired hypohyals and large ossified 

 ceratohyals. But the hyomandibular takes no share in the support of the 

 jaws. It disappears, indeed, entirely in the Dipneumones where the cerato- 

 hyals alone remain, and, as Huxley showed, is represented in Ceratodus by a 

 minute vestigial cartilage, overlying the hyomandibular branch of the seventh 

 nerve. 



The skull of Ceratodus has a continuous dermal temporal roof as in 

 the stegocephalians. Parts of the skull resemble the cartilaginous struc- 

 ture in the urodele embryo, 

 especially in the region of 

 the arches, but here the re- 

 semblance stops. 



In the arrangement of the 

 jaw muscles this dipnoan is 

 very simple. The skull con- 

 sists of a massive cartilag- 

 inous part with a covering 

 of dermal bones which gives 

 the head its large size. The space between the cartilage surrounding the 

 brain and the dermal bone is filled with the enormous temporal muscle 

 which supplies the motive power for the great crushing apparatus. The 

 general aspect of the head is amphibian-like to an extent found in no 

 other fish but the eels. There is a space along the mid-dorsal region of 

 the cartilaginous skull that is covered with the temporal muscle and with 

 the muscles of the neck region. In Polypterus and Amia the muscles of 

 the skull top make no approach to this extension in the dorsal region, but 

 in the eels the temporal muscles extend up and the muscles of the two 

 sides meet in a reptilian way. The musculature of Neoceratodus, in fact, 

 represents a highly specialized dipnoan type, retaining very little of the 

 primitive fish type. The loss of the hyomandibular causes the loss of 

 several muscles common to the elasmobranchs and the teleosts. 



Figure 1 

 Mandible of Neoceratodus forsteri with the tendi- 

 nous fascia of the adductor mandibular muscle 



