1 ) ERMA L SKELE TON 



213 



The bones covering the head consist essentially of a paired 

 series of frontals between the orbits, nasals in front and parietals 

 behind. Circumorbital bones surround the eye-sockets ; a trans- 

 verse row of small bones (often called supratemporals) covers 

 the occipital region, and forms a transition between the cranial 

 plates and the scales of the body; and the sides of the cranial 

 roof are completed over the auditory region by paired pterotics 

 (squamosals) overhanging the articulation of the jaws. The upper 

 jaw is bounded on either side 

 by a toothed premaxillary and 

 maxillary. Each ramus of the 

 lower jaw is provided with an 

 outer toothed dentary and pos- 

 terior lower angular ; to these 

 should be added a dermal articular 

 and a coronoid (complementary). 

 The dermal articular plate seems 

 to be the last element of a series 

 of infradentaries found in certain 

 early forms (Megalichthys, etc.). 

 Ventrally the space between the 

 rami is covered with gular plates, 

 of which a row extends upwards 

 on either side on to the hyoidean 

 opercular flap, where they expand 

 into the subopercular and oper- 

 cular bones. 



On the roof of the buccal 

 cavity are a median posterior 

 parasphenoid underlying and 

 supporting the basis cranii, and 

 paired anterior vomers. Large 

 pterygoids and smaller palatines 

 cover the palato-quadrate arches, 

 which do not form the biting- 

 edge of the jaw, and are usually 

 separated in the middle line in 

 front. A splenial on either side 



lines the inside of the lower jaw. All these internal bones may 

 lie toothed, and are indeed to a great extent formed by the 

 cementing together of numbers of small teeth (p. 217). Similar 

 bony plates with teeth may be found on the gill-arches. To this 

 list of primitive cranial dermal bones should perhaps be added a 

 median ethmoid in front, a median occipital behind, and a median 

 gular below. 



Limiting the branchial chamber behind is the dermal shoulder- 



Fig. 180. 



Diagram of a section through the dorsal fin 

 of Lepidosteus. a, actinotrich ; h, scale ; d, 

 denticle ; I, lepidotrich ; r, radial. (From 

 Quart. Jouni. Micr. Set'.) 



