338 



TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



illustrated in a large skull of Hemiiripterus americanus. Here (Fig. 2155) all the bones of 

 the skull roof, braincase, jaws, etc., are so thin that the growth zones and trabeculae may 

 readily be seen with the naked eye. The cranial spikes are seen to have definite relations 



Fig. 218. Prionotus. 



with the growth-centers. In general, radiating fibres predominate in the ectosteal bones, 

 while zonal growth predominates in the endochondral bones of the braincase, suspensorium 

 and primary upper jaw. Three beautiful triradiate sutures are seen on the occipital surface; 



an^ cf^ pop 



Trigia hirundo 



Fig. 219. Trigia. The epiotic is seen only in the occipital view. After AUis. 



