GREGORY: FISH SKULLS 



133 



primary upper jaw as in teleosts and each bears a single elongate supramaxilla. Tate 

 Regan (1923a, p. 456) suggests that they have abstracted this element from the outer 

 circumorbital plates in both Caturus and Eugnathus, as well as in Amia. Possibly this 

 has happened in response to the pull on the maxilla of the superior branch of the adductor 

 mandibulse muscle. 



On the cranial roof the frontals have assumed the importance that is characteristic 

 of teleosts, while the parietals are shortened anteroposteriorly. The other surface elements 

 also approach the primitive soft-rayed teleost type. 



The skull of Caturus with its broad smoothly-rounded operculars and clean-lined 

 contours foreshadows the mackerel type, as also in the progressive thinning of the surface 

 plates and scales, but these resemblances are far more probably due to convergence than 

 to direct phylogenetic relationship. The skull-roof of Caturus has been figured by A. S. 

 Woodward (1897, PI. VIII). It is flat and Jmia-Vike, with very large frontals and small 

 parietals, which however meet in the midline, the bony supraoccipital not yet having 

 made its appearance on the skull-roof. 



In the more advanced amioids such as Hypsocormus the skull foreshadows that of Amia 

 in the fact that the posterior circumorbitals are already much elongated anteroposteriorly, 

 although very irregular in contour. The postorbital series of cheek plates, however, is 

 greatly enlarged, a point of wide contrast with Amia. 



scdlebone 

 ^(exfra scapu.la.rj 



pf &pareth 



deth-ros 



antorb- 

 (adn) 



cit 



Amia calva 



p*y^ 



Amia calva. Side view. 



Amia. — In Amia (Figs. 235, 28) the first circumorbital series has become greatly 

 enlarged, while the cheek plates (postorbitals) have either entirely disappeared or at most 

 are represented by several irregular ossicles, apparently first noted by Shufeldt and recog- 

 nized by A. S. Woodward (1895, p. 369) as being the representatives of the cheek plates 

 of the older ganoids. 



Taken as a whole, the skull of Amia has advanced far toward the primitive teleost 



