100 



COMPARATIVE ANATOMY 



bone, the paraquadrate (Gaupp), usually described as a squamosal. 

 The quadrate, exoccipital, prootic, orbitosphenoid, and columella 

 arise in the perichondrium and are replacing bones, while all the 

 others are investing bones. 



The temporal region is either uncovered by skeletal parts, 

 or an upper zygomatic bar (supratemporal arcade) is formed by 

 processes of the paraquadrate and frontal respectively, and indi- 

 cates a reduction of a more marked development of bone in this 

 region such as occurred in the Stegocephali. 



In connection with the lower jaw are usually developed a 

 replacing articular at the proximal end of Meckel's cartilage, and 

 investing splenial and dentary bones. The rest of the visceral 

 skeleton of Urodeles undergoes various modifications in the 



gu. 



mk, 



FIG. 73. SKULL AND VISCERAL AKCHES OF Ah Koroma. From the side. 



I, mandible ; II, hyoid ; III- VI, branchial arches ; qii, quadrate, covering which 

 is the paraquadrate (" squamosal") ; ar, articular; ml', Meckel's cartilage 

 enclosed by the dentary bone. 



different types. We may consider the ground-form, as exhibited 

 in the larva, to consist of five pairs of bars in addition to the 

 mandibular arch (Fig. 73), in which latter the palatoquadrate and 

 Meckel's cartilage chondrify independently. The anterior bar, or 

 hyoid, consists of two pieces (Fig. 74. A), as do also the two first 

 branchial arches. The third and fourth branchial arches are much 

 smaller, and even vestigial in Salamanders. All these bars are 

 connected with a single or double basal piece. At the close of 

 larval life, that is, when the gills are lost, the two hinder pairs of 

 arches disappear entirely, while the two anterior pairs undergo 

 changes as regards form and position, 1 and may become more or 

 less densely ossified (Fig. 74, B D). 



Gymnophiona. In contrast to the extensive and compact 

 chondrocranium of most Urodela and of Anura, that of 

 the limbless Amphibians consists of delicate cartilaginous rods 



1 In the genus Spelerpes, which possesses a sling-like tongue, the dorsal 

 segment of the first branchial arch grows out into a long cartilaginous filament, 

 which extends far back under the dorsal integument (Fig. 74, D). 



