350 ■ CHORD AT A. 



bones which thus belong to the visceral arches. In con- 

 nection with these visceral arches are the premaxillce and 

 inaxillce forming the upper jaws and carrying a single row 

 of small teeth. From the hind- end of the maxilla is a small 

 bone (the quadratojugal) which unites behind with the 

 quadrate. 



In connection with the suborbital bar are paired pala- 

 tines anteriorly and paired pterygoids of a triradiate shape. 

 On the dorsal side a pair of T-shaped squamosals overlie the 

 quadrate cartilages. A rod-like cartilage runs from the tym 

 panum to the auditory capsule ; it is called the columella 

 and probably corresponds to the hyomandibular of the skate. 

 The lower jaw, as in the skate, is formed of the mandibular 

 cartilage^ but it has also three bones. It bears no teeth. 



A large plate of cartilage, the lingual plate (or hyoid 

 cartilage)^ rests below the tongue ; it has two long anterior 

 cornua, which are attached to the auditory capsule, and the 

 posterior cornua which are shorter. It is the /^jf^/^s' cartilage, 

 with perhaps a single pair of branchial arches {posterior 

 cornua). 



At first sight there is little in common between this skull and that of 

 the skate. If, however, we carefully follow the following modificaiions 

 which have probably taken place, the comparison is easier. Let us 

 suppose that the palatoquadrate cartilages of the skate become fused on 

 to the nasal region anteriorly and to the auditory region posteriorly, and 

 that further these cartilages are bent out laterally so that they lie no 

 longer under the cranium but round the outer border of the eyes. A 

 condition is thus produced closely similar to the cartilaginous cranium 

 of the frog. The cartilage is then replaced by bone in parts, producing 

 the CARTILAGE-BONES, spheuethmoid, prootics and exoccipitals. Lastly, 

 this skull is covered up by a number of membrane-bones, f^'^AXQ^ fronto- 

 parietals and nasals above, parasphenoid and vomers below, and a 

 number of others, palatities, pterygoids, squamosals, quadratojugals, 

 premaxillce and maxillce, in connection with the visceral arches. 



The vertebral column in the frog consists of nine free 

 vertebrae and a urostyle. The vertebrae are ossified and 

 form rings. The first or atlas is a simple ring with two 

 facets for articulation with the skull. The second to seventh 

 are procoelous, i.e., they articulate with each other by a 

 concavity in front and a convexity behind. The main 

 portion of the vertebra is called a centrum and above this is 

 a neural arch covering over the spinal cord. A large lateral 

 process on each side consists of a transverse process which 



