GREGORY: FISH SKULLS 



79 



In the earliest chordates the orobranchial cavity was doubtless already surrounded 

 with sensory projections and the "head" was only the synthesis of paired sense organs and 

 their cranial nerves, which were all designed either to direct the mouth toward the food 

 or to suck the food into the mouth. Hence the building up of a syncranium of the teleost 

 type was only a further development of an integration that was already well begun in the 

 ostracoderms. 



Serial Homology of the Jaws with the Gill Arches 



In some of the anaspid ostracoderms the mouth cavity was protected externally by 

 thin dermal plates, but if cartilaginous jaws were present they were not sufficiently calcified 

 to be preserved in the fossils and hence the ostracoderms are often referred to as Agnatha, 

 or jawless chordates. However this may be, Kiaer (1924) has shown that in certain of these 

 anaspids in some features the arrangement of the dermal plates on the outer surface of the 

 orobranchial chamber is suggestive of the dermal jaws of true fishes. At this stage the 

 gill-openings were small and circular, as in existing cyclostomes. 



idhy add' id' adore trpz. ^^s- 



m 



ad-arc ^ 



leir.labsup 

 proavgor lab'Urt ^^^^^^ 



Fig. 4. Skull and visceral arches of Chlamydosetackus, with the deep muscles of the branchiocranium. 

 Based chiefly on the plates and descriptions of Allis (1923a), supplemented by a partial dissection of a specimen collected by 



Dr. Bashford Dean. ■ i j 



The deep muscles fall into two main groups: extensors of the oral and branchial arches, running anteroposteriorly, and 



flexors, running vertically. 

 Abbreviations (Allis): 



ad. arc. = musculi adductores arcuales 1-6. id hy = interarcualis between hyal and first branchial arch. 



ad. d. = musculi adductores dorsales 1-5. lab cart = labial cartilages. 



ad. mand. = musculus adductor mandibula;. lev lab sup = musculus levator labii supcrioris. 



care = musculus coraco-arcualis. lev mx sup = musculus levator maxillae superioris. 



cb = musculi coracobranchiales 1-6. pal qu = palatoquadrate. 



CO sc = coracoscapular arch. pro ang or = musculus protractor anguli oris. 



hyom = hyomandibular. trpz = musculus trapezius. 



id = musculi interdorsales 1-S. 



A comparison of the orobranchial region of an ostracoderm (Fig. 2A) with that of a 

 typical gnathostome (Fig. 4) reveals, however, a striking difference. In the ostracoderms 

 the mouth is much greater than the gill-openings and widely removed from them. In the 

 primitive gnathostome, on the other hand, the mouth is in series with the gill-slits and only 



