Explanation of Plate IV 



Jaw muscles of Polypterus, representing the Crossopterygii, and of 

 Neoceratodus, representing the Dipnoi 



Figs. 1 and 2.-*-Polypterus. (Muscles mainly after Pollard.) 



The deep muscles are shown in Fig. 1, after the removal of the preoper- 

 culum, and the superficial muscles in Fig. 2. 



The superficial layer of the adductor mandibular (Adm. 1 ) extends from the 

 enlarged hyomandibular to the ascending ramus of the mandible. The deep 

 branches (Adm. 2 , Adm. z ) run nearly at right angles to the outer branch and 

 arise from the side of the skull. This arrangement foreshadows the differen- 

 tiation of the jaw muscles in amphibians into masseter, temporal and ptery- 

 goid branches (Pollard). The levator maxillae superioris (L.m.s.) is closely 

 associated with the protractor hyomandibularis (P.hy.) as in elasmobranchs. 

 A small slip of the latter muscle runs dorsad to the spiracular ossicle. The 

 dilator operculi (D.o.) seems to be absent. In Fig. 1 the hyomandibular is 

 seen with the area of origin of the adductor mandibular on its anterior border. 

 The levator operculi (L.o.) is closely associated with the adductor hyoman- 

 dibularis (Ad. hy.). 



Fig. 3. — Neoceratodus. Side view of jaw muscles. 



The superficial layer of the adductor is fastened posteriorly to the opercular 

 region. The middle layer (Adm.-) runs over the top of the massive chon- 

 drocranium and separates the dermal skull roof from it. This part of the 

 adductor is divided into numerous small fascicles interspersed with connective 

 (issue. 



Fig. 4. — Neoceratodus. Viewed from above. 



