526 CHORDATA. 



quadrate-articulare joint. It should be said that another view obtains, 

 though not so well supported, which considers the ear bones as exactly 

 homologous throughout terrestrial vertebrates, and which recognizes incus 

 and malleus in the columella and maintains that quadrate and articulare 

 form the hinge of the mammalian jaw. 



In conclusiou three other bones, widely distributed, must be 

 mentioned — the squamosal, the tympanic, and the jugal. The 

 squamosal is a membrane bone arising at the boundary of quadrate 

 and otic capsule (j^etrosal), and hence with relations to both these 

 bones. It increases in size as the quadrate diminishes in changina; 

 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 i^etrosal, it forms a frame for the 

 attachment of the tympanic membrane of the ear. The jugal 

 (malar, zygomatic) belongs to the maxillary series. In many ver- 

 tebrates this series 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 gaj) 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 second- 

 ary bones touch, especially since in the latter no general criteria of 

 distinction can be drawn. Thus the pterotic, sphenotic, and ectethmoid of 

 the fishes are often replaced by other secondary bones in the Amniotes ; 

 the primary pterotic by the secondary squamosal ; the primary 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, jiaired and unpaired, which 

 generally occur together only in fishes (figs. 59S-G08). The un- 

 paired 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 sub- 

 divided into smaller fins), a caudal fin, and an anal fin. In a 

 similar way, apparently, tlio paired appendages — an anterior or 

 thoracic and a posterior or pelvic pair — have arisen from a pair of 

 continuous folds, by develojimeut of the ajipendages themselves 



