Explanation of Plate IV 



Jaw muscles of Polypterus, ropresenting the Ci'ossopterygii, and of 

 Ncoceraiodiis, represc-nting the Dipnoi 



Fics. 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.^) extends from the 

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

 branches {Adm.-, Adm.^) 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 maxillie superioris (L.m.s.) is closely 

 associated with the protractoi- 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 mandibulse on its anterior border. 

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

 dibularis {Ad. hy.). 



Fig. o. — Ncoccratodus. Side view of jaw muscles. 



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

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

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

 adductor is divided into numerous small fascicles interspersed with connective 

 tissue. 



Fig. 4. — Neoccratodus. Viewed from above. 



