462 CHORDATA 



According to this view the lower jaw of a mammal is not exactly equivalent 

 to the lower jaw of a bird, since in the latter 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. 



In conclusion three other bones, widely distributed, must be mentioned 

 the squamosal, the tympanic, and the jugal. The squamosal is a mem- 

 brane bone arising at the boundary of quadrate and otic capsule (petrosal), 

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

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

 with the petrosal to form the temporal bone. In common with the tympanic, 

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

 attachment of the tympanic membrane of the ear. The jugal (malar), 

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

 is articulated only in 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 and the quadrate region of the skull. 

 When the quadrate becomes modified to the incus, the jugal articulates 

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

 forward for this purpose. 



Difficulties in ascertaining the morphological relations of bones arise where 

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

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

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

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

 prefrontals and postfrontals 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 unpaired consist of a fold of the skin beginning 

 in the sagittal plane behind 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 fin (often subdivided 

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

 ently, the paired appendages an anterior or thoracic and a posterior or 

 pelvic (abdominal or ventral) pair have arisen from a pair of continuous 

 folds, by development of the appendages themselves and suppression of 

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

 since they occur not only in the cyclostomes, but in AmpJiiovus and the 

 tumcates 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- 



