462 CHORDATA 



According to this \'ie\v the lower jaw of a mammal is not exactly equivalent 

 to the lower jaw of a bird, since in the lauer the hinge is furnished by the quad- 

 rate-articulare joint. It should be said that there is another view, though not 

 so well supported, which considers the ear bones as exactly homologous through- 

 out terrestrial vertebrates. 



lit conclusion three other liones, widely distributed, must be mentioitod 

 — the squamosal, the t\'ntpanic, and the jugal. The siiiiaiiiosal is a mem- 

 brane lioite arisiitg at the boundary of quadrate and otic ca]isule (petrosal), 

 and hence with relations to both these bones. It iitcreases in size as the 

 quadrate tlimiitishes in changing to the incus, and in the mammals fuses 

 with the petrosal to form the Icniporal bone. In common with the /ympanic, 

 ■which in mammals also fuses with the petrosal, it forms a frame for tlie 

 attachment of the tympanic membrane of the ear. The iiii;al (malar), 

 belongs to the maxillary series. In manv vertebrates the maxillary bone 

 is articulated only iit front, its posterior end terminating freely in the soft 

 parts, but when the jugal occurs it forms a jugal or zygomatic arch which 

 bridges the gap between the maxillary aitd the quadrate region of tJie skull. 

 Wheit the quadrate becomes modified to tlte incus, the jugal articulates 

 with its companion, the squamosal, wdiich extends a zygomatic process 

 forward for tliis purpose. 



Difliculties in ascertaining tlic morphological relations of bones arise where 

 the visceral and cranial parts join and where primary and secondary bones 

 touch. Thus the pterotic, sphenotic, and ectethnioid of fishes are often replaced 

 by secondary bones in the Amniotes; the pitcrotic by the squamosal ; the sphenotic 

 and ectethnioid by two membrane bones in front of and behind the frontals, the 

 frcjrontah and postfroiitah of reptiles and other forms. 



Just as skull and vertebral column form a firm axis for the body, the 

 appendages are supported by axial skeletal structures. Two kinds of 

 appendages are recognized, paired and unpaired, which generally occur 

 together only in fishes. The uitpaired consist of a fold of the skin beginning 

 in the sagittal plane behiiul the head, running back around the tail and 

 forward on the ventral surface to the anal region. This continuous 

 fold is nearly always divided into three parts, a dorsal fni (often subdivided 

 into smaller fins), a caudal Jin, and an anal Jin. In a similar way, appar- 

 ently, the pairetl appendages — an anterior or llioracic and a posterior or 

 pelvic {abdominal or vciilral) pair — ha\'e arisen from a pair of continuous 

 folds, by development of the appenilages themseh'es and suppression of 

 the intermediate regions. Of these the unpaired are possibly the oldest, 

 since they occur not oidy in the cyclostomes, 1nit in Ainphioxns and the 

 tunicates as well, where paired appendages are lacking; on the other hand 

 they disappear in the higher forms. Since they are of service only in an 

 aquatic life, they are lost in Amphibia, in which a continuous fin, unsup- 



