GREGORY: FISH SKULLS 



107 



margins of the jaws and a few very large tusks on the dentigerous plates of the primary 

 upper jaw and on the coronoid bones of the mandible; the large but feebly ossified hyoman- 

 dibular is inclined backward and then sharply forward (Watson and Day, 1916, p. 16); the 

 skull is rather low, with a blunt, rounded rostrum, eyes small and far forward; all the super- 

 ficial bones, scales, etc., are covered in the most primitive genera with a shining armor of the 

 "cosmoid" type, as described by Pander (1858) and Goodrich (1908); the nares, at least in 

 some types, have an internal opening in the palate, as in primitive Stegocephalia. 



The orbit is surrounded by a variable number of circumorbitals, homologous as a whole 

 with those of primitive tetrapods. The cheeks are covered originally by two broad plates, 

 apparently corresponding to the squamosal and quadratojugal of tetrapods and possibly 

 in part with the "postorbitals" or cheek plates of semionotids (see p. 126 below). The pre- 

 opercular is small (Osteolepis) (Goodrich, 1919), carrying the hyomandrbular branch of the 

 lateral-line canal, but sometimes covered by cheek plates. The interopercular and branch- 

 iostegals of higher fish are probably represented by so-called "lateral gulars." A row of 

 infra-dentaries corresponds with the surangular, angular, postsplenial and splenial of the 

 oldest amphibians (Watson, 1926). 



The skull top presents the following conspicuous features: surface of rounded rostrum 

 either more or less continuous {Osteolepis) or subdivided into numerous paired and unpaired 

 plates (Dictyonosteus); a pineal foramen located between the short frontals {Osteolepis); 

 parietals elongate. In the osteolepids the whole fore part of the skull could probably be 

 turned upward, as a deep crease extends behind the upper jaws and cheek and behind the 

 frontals, while the endocranium as a whole consists of two distinct parts, the rostro-orbital 

 piece, including the vomer and short parasphenoid, being separated by an unossified space 

 from the occipital region (Bryant, Watson). This is widely unlike the chondrocranium of 

 primitive actinopterygians and its morphogenetic relations thereto are not well understood, 

 although the facts themselves as to the osteology of the cranium of Palaeozoic Crossopterygii 

 of several genera are well known, through the labors of R. H. Traquair, W. L. Brj-ant, 

 D. M. S. Watson and Eric A:son Stensio. 



pmx 



. pmx 



pas 



Fig. 9. 



Palatal view of skull of Devonian crossopterygian (A), and Ix>wcr Carboniferous amphibian (B). 



A. Eusthtnopttron. From Watson, mainly after Bryant. 



B. Baplieles kirkhyi. After Watson. 



