internasal plates 

 nasal series 



nasal opening 



lateral rostral 



supraorbital canal 

 parietal foramen 



anterior pit line 



postspiraculo 



lateral extrascopular. 

 medial extroscapula 



OSTEOLEPIS 



EUSTHENOPTERON 



Figure 4-29. Skull roots of an osteolepid (Osteolepis) and a rhizodontid Eusfhenopteron, repre- 

 sentatives of the two families of Osteolepiform fishes. (After Jarvik, 1948) 



sented in the crossopterygian by a series of "nasal" plates 

 along the sensory canal and an irregular group of internasals 

 (Figure 4-29). Primarily on the basis of the position of the 

 parietal foramen, Westoll (1941) suggested the homologies 

 of the bones in these two groups which are used in this dis- 

 cussion. 



The roof of the skull appears to be hinged between the 

 parietals and postparietals. The cheek is separated from 

 the roof by a spiracular cleft; a postspiracular bone is 

 wedged between the supratemporal and tabular. 



The osteolepiform had a large operculum covering the 

 gill openings (Figure 4-28). The operculum was supported 

 by an opercle and subopercle. The interramal area of the 

 throat was covered by gular plates. The dermal bones of 

 skull and jaw contained sensory canals from which short 

 tubes, somedmes numerous and branching, extended to the 

 surface of the bone. 



The endocranium (as described for Eusthetwpleron, Figure 

 4-30) was divided into two parts along a line passing 

 through the foramen of the profundus division of the trigem- 

 inal nerve and below the parietal-postparietal suture of 

 the dermal roof. Since this joint is not movable, identifi- 

 cation of it as a hinge is misleading. The anterior part of 

 the endocranium is the ethmosphenoid, the posterior divi- 

 sion the oticoccipital. The posterior division is subdivided 

 by a fissure, actually a synchondrosis which may be bridged 

 ventraily by thin ossifications, into an otic and an oc- 

 cipital segment. The occipital segment enclosed the fora- 



men magnum above and the notochord canal below. It ex- 

 tended forward between the otic capsules from which it was 

 partly separated by an ovoid vestibular fontanelle. The 

 jugular foramen opened between this segment and the otic 

 capsule 



The otic capsules were joined by a synotic tectum in 

 which was a small posterior fenestra. Just above the otic 

 capsule were hollows in this roof; these are the fossae 

 Bridgei. Each fossa was margined laterally by a pterotic 

 wing. On the lateral aspect of the otic segment was a large 

 lateral commissure with grooves anterior and posterior to 

 it. This commissure formed the outer wall of a chamber into 

 which opened the facial foramen. The lateral head vein 

 also passed through this chamber The posterior portal was 

 used by the hyomandibular division of the facial nerve. 

 Above and below this portal were articular areas for the 

 hyomandibula. 



The ethmosphenoid division of the endocranium con- 

 tained the anterior part of the brain cavity and the olfactory 

 nerve canals. Posteriorly it consisted of the orbitosphenoid 

 ossifications fused across the midline above and below. There 

 was a wide parietal canal opening in the roof and a midline 

 cerebral fenestra, filled by cartilage, posteriorly. The poste- 

 rior orbital margin was somewhat raised and culminated 

 above in a basipterygoid process, which lay behind the 

 pituitary vein foramen. 



The hyomandibula (Figure 4-31) had two articular facets 

 proximally, an anteroventral one and a poslerodorsal one. 



94 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



