GREGORY: FISH SKULLS 12S 



years or so of piscine history. The chondrostean series appears in the Middle Devonian 

 and dominates the inland waters through the later Paleozoic, thereafter giving way before 

 the higher ganoids and their descendants and being represented today only by the specialized 

 and in some respects depauperate sturgeons and spoonbills. The Holostei, or Protos- 

 pondyli, first appear in the records in the closing epoch of the Paleozoic, become dominant 

 in the early Mesozoic and then give place before the increasing hosts of their own descend- 

 ants. At present the Protospondyli are represented by the garpikes (Lepidosteidae), a 

 specialized branch of their oldest stage, and by Amia, an offshoot from the later Protospon- 

 dyli that stands near the base of the teleost stem. 



Semionoiids. — The first of the Holostei, or Protospondyli, was the Permian Acentro- 

 phorus (Fig. 215), which has the characters typical of the family Semionotidae. In these 

 fish the body is short and deep and so is the head, which, as described by E. L. Gill, con- 

 forms to the body contours. The skull pattern contrasts widely with that of the palaeonis- 

 cids: the mouth is small and of the nibbling type with delicate styliform teeth; the sus- 

 pensorium is vertical or inclined forward; the eye is large, located partly behind and above 

 the very small upper jaw. In addition to the sclerotic plates there are two rows of large 

 orbital plates, called here the circumorbitals and the postorbitals, the latter more or less 

 parallel to the curved opercular and branchiostegal series. Above the eyes there are several 

 supraorbital plates. The more or less concentric arrangement of the bones of the face in 

 these fish, while contrasting widely with the oblique arrangement of the cheek-plates of 

 the Chondrostei appears to be one of the "basic patents," so to speak, for the skulls of all 

 higher fishes. The probable origin of this arrangement is discussed below. 



The same type of skull pattern (Fig. 22) is preserved in the Triassic and later genera, 

 Dapedius, Semionotus, Lepidotus and their relatives. All are more or less orbicular to 

 fusiform in body, with small nibbling mouths and concentric arrangement of circumorbitals, 

 postorbitals, etc. The circular arrangement thus seems, in fact, to be especially correlated 

 with an orbicular body-form and small mouth. Hence when later in longer-bodied fish we 

 find clear traces of the circular pattern, the inference seems probable that such body-forms 

 have been derived by lengthening of a sub-orbicular type, as has clearly been the case in 

 several other families (Pholidophoridse, Characidae, Carangidae). All bony plates of the 

 typical teleost skull are present and in addition others that become reduced or rare, es- 

 pecially in higher teleosts. In this category belong the several extra elements in the inner 

 or circumorbital series, the entire row of "postorbitals," the antorbitals and the several 

 sheathing bones (so-called splenials and coronoids) on the mesal side of the mandible. 



Most of the elements in the "circular" pattern of the skull can be identified readily 

 with those of the oblique or asymmetrical pattern of the primitive palaeoniscoid skull 

 (Fig. 12). Nevertheless it is difficult to be sure in certain cases, especially where the num- 

 ber of elements in each of the series is different in the two gfoups, as in the circumorbital 

 and postorbital series. 



The circular pattern may be supposed to have been derived from that of the palaeonis- 

 coids, partly as a result of the marked shortening of the jaw and the consequent forward 

 migration of the joint between the mandible and the quadrate. This would drag the at- 

 tached subopercular downward, perhaps causing the lower end of the subopercular to be 

 fractured and pulled forward out of its place to form the interopercular, which now appears 

 for the first time as distinct from the subopercular (Tate Regan, 1929, pp. 31, 313). The 



