78 ANNALS NEW YORK ACADEMY OF SCIENCES 



POLYPTERUS 

 Plate IV, Figs. 1, 2 



The condition of the skull in Polypterus is extremely primitive, as all 

 the dermal elements are on the surface. The skull is broken up into 

 small elements so that the comparison with the Devonian Ehipidistii, 

 such as Osteolepis, is easily made. The elements may not be exactly the 

 same, but they represent the breaking up of the hard dermal shell to 

 allow the necessary movement occasioned by the muscular development. 

 It might be possible to account for all of the breaks by making a phylo- 

 genetic study of the muscles in the fossil forms. In this connection 

 Gregory (1915, p. 327) offers the following very interesting and sug- 

 gestive hypothesis : 



It may be stated as a general hypothesis that in the dermocranium of the 

 primitive fishes the position and arrangement of the sutures and the subse- 

 quent pattern of the osseus elements are the evolutionary resultants of the 

 various symmetrically balanced stresses induced by the action of the under- 

 lying muscles of the eyes, jaws, branchial arches and pectoral limbs, in com- 

 position with the position and size of the olfactory, optic and auditory cap- 

 sules. It is at least a fact that sutures and articulations define loci of greatest 

 mobility, centers of ossification define loci of least mobility. Differential 

 growth of one region of the skull, as in the rapid elongation of the snout, also 

 results in more or less rearrangement of the sutures and osseus elements. 



An examination of the skull of Polypterus shows that Dr. Gregory's 

 hypothesis works out well, as most of the breaks in the skull might be 

 explained by the stress of the muscles actually present. The adductor 

 mandibular would tend- to make the break over the frontal and parietal 

 region, as this is at right angles to the pull. The pull of the masseter 

 would cause the break in front of the preoperculum by its pull against 

 the squamosal and quadrate elements. The spiracle and its muscle might 

 account for the broken condition in that region with its many small 

 plates. The breaks in the region in front of the orbits are more difficult 

 to account for, but in the sharks and some of the other fishes there is a 

 levator maxillae which might have caused the breaking up in this region. 



The jaw muscles of Polypterus have been figured and described by 

 Pollard (1892). 



MUSCLES OF TB?E ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V s ) 



Adductor mandibular (Ad. m.). 

 Protractor hyomandibularis (P. h.). 

 Levator maxilla? superior is (L. m. s.). 



