420 OSBURN 



V. The pelvic girdle cannot be strictly homologized with 

 the pectoral, point for point, for the following reasons: 



(i) In the oldest fossil sharks in which the pelvic is sufficiently 

 known (Pleuropterygidae) there is no pelvic girdle developed 

 beyond the condition of basalia. If the gill-arch theory were 

 true, the pelvic girdle should be best developed in the oldest 

 forms. 



(2) In the lowest modern sharks (Notidanidas) there is in 

 the adult no evidence of a dorsal prominence in the pelvic to 

 correspond with the scapular portion of the pectoral. It is 

 difficult to see how the pelvic girdle of Chlamydoselachus, a 

 long, flat plate pierced with eight nerve foramina, could be 

 made to homologize with the pectoral girdle of any shark. 



(3) In none of the stages of Chlamydoselachus in my pos- 

 session in which the pelvic is sufficiently developed (from no 

 mm. upward) is there any indication of a dorsal prominence. 

 The pelvic girdle develops as a flat basale-like plate. 



(4) The argument for the anterior prominence which is 

 present in some sharks and which was originally homologized 

 with the scapula by von Davidoff has already been given up by 

 the gill-arch theorists. The small dorsal prominence recently 

 described (Braus '04) in the pelvic of Spinax and homologized 

 with the scapula can scarcely be considered homologous, for 

 the reason that it is situated posterior to the nerve foramen, 

 while the scapular portion of the pectoral girdle is always, so 

 far as my observations have extended, anterior to the foramen. 



The various parts of the pelvic girdle cannot, therefore, be 

 homologized with all parts of the pectoral, and certainly the 

 pelvic girdle is much farther removed from comparison with 

 the gill arch. 



B. It becomes evident that no migration yet shown is quan- 

 titatively sufficient to account for the distance between the 

 pelvic fin and the gill region, for: 



I. The slight measurable migration which the pelvic may 

 undergo in its ontogeny cannot be accepted as evidence in favor. 

 of migration from the branchial region. 



(1) The pelvic fin in some cases migrates forward during 

 ontogeny. 



