hyomandibula articulation area 

 otic branch of VII 



lateral ethmoii 



bosisphenoid 

 A myodome' 



^^ basipterygoid process 



hyoid efferent 



carotid canal 



fossa Bridgei 



lateral commissure canal 



occipital division of endocronium 



efferent pseudobranchiol artery 



^spinoccipital foramina 



palatine branch VII- 



hyomondibular articulation 

 hyomandibular branch VII 



efferent I- 



_lateral ethmoid 



hypophyseal fenestra 



basipterygoid process 



:i- 



phoryngobranc 



hial 



B 



(V^suprabranchial I 



epibranchial I 



efferent I 



■^spinoccipital foramina 



foramen magnum 



hypobronchial 



cerotohyal anterior 

 ceratohyal posterior 



hyomandibular articulation 



hyomandibular branch VII 

 ■ceratobronchiol I 



C 



lateral canal 



notachord canal 

 efferent I 

 efferent II artery 



Figure 5-12. Endocronium and viscerol skeleton (except jows) of Pteronisculus. A, lateral view of 

 endocronium; B, ventral viev/; C, posterior view; D, loterol view of hyoid and first broncliial orcfies 

 (quadrate sfiown in relation to hyomondibulo and sympletic). (After Nielsen, 1942) 



ences might be attributed to the jaw apparatus. The suspen- 

 sorium, as represented by the hyomandibula, was directed 

 posteroventrally, with the articulation process of the quad- 

 rate well behind the vertical of the eye. The cheek was 

 solidly covered. There was a well-developed dermohyal 

 between the operculum and preopercle, and sometmies 

 there were additional plates behind or below this. The bony 

 operculum was relatively narrow, in an anteroposterior 

 direction, and continued onto the throat as a ray-supported 

 membrane; there was never an interopercle, although a 

 large plate of branchiostegal origin sometimes lay below 

 the subopercle. The intertemporal was included in the roof. 

 In at least one genus (Moythomasia), there was a parietal 

 foramen. The snout had a large internasal, large lateral 

 nasals, which margined the eye (except in Cheirolepis), and 

 generally lacked separate medial rostrals— these were usu- 

 ally fused with the premaxilla, though both were sometimes 

 lacking. The eye was proportionally large, with four scle- 

 rotic plates, and the snout short. 



Among what have been called palaeoniscoids, there was 

 marked divergence from this picture. The suspensorium, as 

 an extreme, was vertical and the cheek was free of bony 

 plates (Dorjpterus). Reduction of many bones in size, or 



even their loss, is observed in several forms. In others there 

 has been a reduction in the number of bones but a reten- 

 tion of a heavy dermal armor; the branchiostegals have 

 been replaced literally or functionally by gular plates in 

 Haplolepis. 



The endocranium (Figure 5-12) was well ossified but had 

 some areas of cartilage. It was composed of occipital and 

 ethmosphenotic divisions separated by a fissure. The basi- 

 occipital region enclosed the restricted notochord canal as 

 well as the aortic canal. There was a large vestibular fon- 

 tanelle between the otic capsule and the basioccipital. 



Relationships among acfinopferygions The actinopte- 

 rygian fishes appear to be characterized by the myodome. 

 In the salmon there are three stages of its development: (I) 

 without a myodome, (2) with a myodome for the external 

 rectus, and (3) with a double myodome, having an addi- 

 tional space, which contains the origins of the other recti, 

 above and behind that for the external rectus. The second 

 stage is possibly the ancestral form since it occurred in 

 many palaeoniscoids. Loss of the myodome in some kinds 

 of ray-finned fishes suggests neoteny, the retention of an em- 

 bryonic stage into the adult. The advanced nature of the 



114 -THE HEAD SKELETON OF FISHES 



