230 BONY FISHES vm. a 



(p. 269) and the two types have obvious similarities, though it is not 

 clear how they are related. 



The skull was built on a distinctly different plan from that of 

 Crossopterygii, in that there was no joint such as was present in those 

 fishes to allow the front part to flex on the hind. The jaw support was 

 amphistylic, in the sense that the palatoquadrate was attached to the 

 neurocranium by a basal process, but the otic process did not reach 

 the skull and the hind end of the jaw was supported by the hyomandi- 

 bula. There were even more dermal bones than are found in modern 

 Actinopterygii, arranged so as to form a complete covering for the 

 chondrocranium and jaws. These bones were derived from the original 

 scaly covering of the head and the naming and comparing them with 

 the bones of other forms is a matter of some difficulty. Some of the 

 main bones resemble in appearance and shape those found in tetra- 

 pods, but there are others for which no such homologues can be 

 found, and sometimes there is considerable difficulty in recognizing 

 even the main outlines of the pattern. The problem is that we have 

 no rigid criterion by which to set about giving names to the skull 

 bones. No system yet discovered is wholly satisfactory, and we must 

 admit to insufficient knowledge of the factors that determine that 

 bone shall be laid down in certain areas and that sutures shall separate 

 these from each other. However, some of the dermal bones lie in 

 relation to the lateral line canals (or rows of neuromasts), which latter 

 may provide the stimulus to bone formation. The lines are remarkably 

 constant, perhaps because of their function in detecting water move- 

 ments in relation to swimming, and this is the factor that determines 

 the position of many of the bones. Others fill in the spaces between 

 (anamesic bones). Yet others may be differentiated in relation to the 

 teeth. However, the number of bones along any one line may vary 

 greatly even in one species (e.g. in Amid). The whole pattern is more 

 variable in fishes than in higher vertebrates, but it is usual to consider 

 that the bones of early Actinopterygii resemble those of Crossopterygii 

 and of the early amphibians (Fig. 194). 



The roof of the skull usually shows a large pair of parietals between 

 the eyes, and post-parietals behind these. Between the parietals and 

 the nostrils there are frontal bones and the front of the head usually 

 also carries a number of rostral bones, not found in higher forms. 

 Behind the post-parietals in the midline is a series of extrascapular 

 bones. 



The side of the skull of palaeoniscids is covered by numerous bones, 

 including a series of pre- and post-frontals, post-orbitals, and jugals 



