Figure 6-56. Menus of Eryops, A, and pes of Trematops, B, o 

 rhochrtomous amphibian; and Copforhinus, C, on early reptile. 



ossifies ft-om what appears to be the tibial center. This 

 modification of the hind limb of the reptile — it is the front 

 limb of the amphibian which is most changed — suggests a 

 difference in limb use. The hind limb becomes the main 

 source of power in the reptile line, leading to bipedalism in 

 some. 



Choanate Tishes 



Pectoral appendage The pectoral girdle of the crossopte- 

 rygian fish consists of six dermal elements. From above 

 down they are the posttemporal, the supracleithrum, the 

 postcleithrum (or anocleithrum), the cleithrum, the clavi- 

 cle, and a small medial intcrclavicle (Fig. 6-57). The first 

 two elements bear the sensory canal connecting the lateral 

 line of the body with the temporal canal of the head. The 

 postcleithrum varies in its relationships from a part of 

 the girdle arc to a more posterior element overlapping the 

 cleithrum-supracleithral joint. The chondral part of the 

 girdle is not well known but is assumed to have a dorsal scap- 

 ular extension, an anteroventral and medial clavicular por- 

 tion, and a tubercle for the articulation of the fin. 



The actinistian agrees fairly well with this. In Laltmena 

 ( Figure 6-58 ) the supracleithrum and intcrclavicle are lacking 

 and an extracleithrum has been added. The extracleithrum 

 appears to be an element unique to this group. The most 

 dorsal unit is perhaps the postcleithrum; however, its exact 

 homology must remain in doubt. In Latimena the fin sup- 

 port process of the scapulocoracoid is partly (perichondrally) 

 ossified. This ossification can scarcely be equated with 

 either the scapula or the procoracoid. In Dipturus this ele- 

 ment has the relationship of a supracleithrum, but in 

 Latimena it could easily be the postcleithrum since it lies 

 deep in the tissues and is not connected with the cranium. 

 It may be that both elements were present and there has 

 been variation in retention. 



The dipnoan as represented by Protopterus (Figure 6-59) or 

 Neoceratodus has a highly modified girdle that lacks the post- 

 temporal, supracleithrum, and intcrclavicle. The postclei- 

 thrum is a bifurcated splint lying in the ligament joining the 



extrascapulors 



posttemporal 



interclavicle 



cleithri 



B 



Figure 6-57. Pectoral girdle of fusfhenopferon as seen laterally. A, from above, B, and 

 from below, C. (After Jarvik, 1 944) 



BILATERAL APPENDAGES • 179 



