APPENDICULAR SKELETON" OF VEETEBEATA. 



479 



Fig. 264. 

 skeleton 



Primary 

 of the 



thoracic fin of Aci- 

 penser rnthenus, 

 after the removal 

 of a portion of the 

 secondary skeleton. 

 B Basale of the nieta- 

 pterygium. R Bony 

 marginal ray of the 

 secondary skeleton 

 of the fin, only 

 figured in part. 



secondary skeleton of the fin-rays (h). Basal pieces can be seen 



in a few only, and it is difficult to refer these even to their 



primitive significance. The arrangements which 



obtain in the Ganoi'dei would lead us to regard 



the basale of the nietapterygiuin, and the basalia 



of some of the rays, as being the most constant 



constituents of these pieces. In consequence of 



their having the same function they have the 



same form, so that it is impossible to show that 



they have any connection with the primary stage, 



except by referring them back to the skeleton of 



the Ganoid fin. 



In many divisions of the Teleostei these 

 pieces undergo great changes, in addition to 

 being diminished in number. They are, for 

 instance, intimately attached to the shoulder- 

 girdle, and immovably connected with the parts 

 of which it is made up (Cataphracti). 



In this way we are able to make out a con- 

 tinuous series from the well-developed skeleton 

 of the fin in the Selachii to that which is found 

 in the Teleostei ; the most important changes 

 consist in the gradual reduction of smaller or 

 larger parts. Keduction first affects the periphery, and then the 

 base, so that the latter is the most constant portion. The decrease 

 in size which the primary skeleton suffers is made up for by the 

 appearauce of ossifications of the integument, which consist, as 

 in the unpaired fins, of jointed or firm bony rays, and are developed 

 on both surfaces of the fin. 



Gegenbaur, C, Untersuchungen zur vergleich. Anatomie der Wirbelthiere. II. 

 Leipzig, 1865. 



§ 363. 



In the skeleton of the fore-limb of the higher Vertebrata we are 

 able to recognise the stem of the archipterygium, with rays attached 

 to one side of it ; no rays are now attached to the shoulder-girdle, 

 the stem only is so attached. The arrangement of the joints of the 

 rays in rows set obliquely to the stem — which is just the 

 same arrangement as that of the primitive rays — is obscured by 

 subsequent transverse jointing, but it can be recognised without 

 difficulty in the lower forms. This jointing gives rise to new pieces ; 

 transverse rows of the joints of the rays, as well as the corresponding 

 joints of the stem, being developed into longer pieces. This change 

 is due to a change in function, in consequence of which the ap- 

 pendage is converted from a swimming organ into a compound system 

 of levers. 



In Ichthyosaurus among the Enaliosaurii the basale of the 

 archipterygium is first of all differentiated from the rest of the 



