APPENDICULAR SKELETON 135 



Perissodactyles, Primates) ; and on each a condylar, a coronoid, 

 and often an angular process (Marsupials, Rodents, Insectivores) 

 may be distinguished. 1 Teeth, which are only exceptionally 

 wanting (e.g. Echidna, certain Edentates), are confined to the pre- 

 maxilla, maxilla, and mandible. They present marked differences 

 in number, form, and size ; together with the muscles, they are 

 the cause of considerable modifications in the form of the jaws 

 and their articulation and may indirectly influence the entire 

 skull, in the study of which the law of correlation must always 

 be borne in mind. 



The hyoid arch (Fig. 95) is connected proximally with the 

 base of the auditory capsule and sometimes becomes more 

 or less ossified, but the greater part is usually reduced to a 

 fibrous band, and may be quite rudimentary ; its dorsal end forms 

 the styloid process of the periotic, and its ventral end the lesser 

 (anterior) cornu of the so-called hyoid bone of the adult. The 

 body of this bone represents the basal parts of the hyoid and first 

 branchial arch, the greater (posterior) cornua belonging to the 

 latter. The hyoid apparatus is connected with the larynx by a 

 membrane, the thyro-hyal ligament, and the thyroid cartilage of 

 the larynx arises in the blastema of the second and third branchial 

 arches. 



V. APPENDICULAR SKELETON 



The problem of the evolution and morphology of the fins and 

 limbs of Vertebrates is one which, in point of interest and im- 

 portance, is comparable to that relating to the head. During the 

 last thirty years it has been attacked vigorously both from the 

 embryological and the palaeontological sides, and has given rise to 

 so many speculations often of a very contradictory nature that 

 only the barest outline of some of the more important results 

 obtained can be given in the course of the present chapter. 



The fins or limbs, which are distinguished from the axial organs 

 (head, neck, and body) as appendicular organs, serve mainly for 

 locomotion, and may be divided into two* groups, the unpaired 

 and the paired (pectoral and pelvic). They arise independently of 

 the axial skeleton. 2 



1 Two or more small bones ("ossa mentalia") occur in Mail between the 

 distal ends of the mandibular rami, with which they unite, taking part in the 

 formation of the mental prominence. 



2 Numerous and varied modifications of the fins occur amongst Fishes to 

 form, e.g. organs for protection, support, attachment, offence, defence, or for 

 alluring prey. 



