FROG: EXTERNAL FEATURES AND BODY-WALL 35 



meets a small membrane bone, the quadratojugal, which 

 connects it with the quadrate. 



The lower jaw or mandible consists of two halves united 

 in front by a ligament. Each half is a curved rod of 

 cartilage, known as Meckel's cartilage, ossified at the end to 

 form the small mentomeckelian bone, and almost completely 

 ensheathed by a couple of membrane bones, the angulo- 

 splenial within, and the dentary without. The latter 

 does not, as its name would imply, bear teeth, the frog 

 having no teeth in the lower jaw. At the near end or 

 angle of the jaw the dentary bears a small knob or condyle, 

 which fits into a hollow, known as the mandibular fossa, on 

 the end of the quadrate. 



The hyoid is a fiat structure in the floor of the mouth. 

 It consists of a wide body with two short processes on each 

 side and two longer processes, the cornua, at each end. 

 The anterior cornua are very long and slender and curve 

 backwards at the sides of the body and then upwards to 

 be attached to the sides of the auditory capsules. The 

 posterior cornua are shorter and stouter and project back- 

 wards at the sides of the windpipe. They are the only 

 ossified parts of the hyoid, the remainder consisting of 

 cartilage. 



The following table represents in a summary form the 

 architecture of the skull : 



Regions of skull. 

 Cranium 



Nasal Capsules 



Auditory Capsules 

 Visceral Arches — 



Upper jaw 



Lower jaw 

 Hyoid 



Cartilage bones. Membrane bones. 



/ Exoccipitals 

 ^Sphenethmoid (part) 

 / Sphenethmoid (part) Nasals. 

 \ Mesethmoid Vomers 



Prootics 



Fronto-parietals. 

 Parasphenoid. 



(Palatines 2 ) 

 (Pterygoids 2 ) 



Mentomeckelians 

 Posterior cornua 



Squamosals. 1 



Premaxillse. 



Maxillae. 



Quadratojugals. 

 f Angulo-splenials. 

 \ Dentaries. 



None. 



1 Attached to auditory capsules but not belonging to them. 



2 Cartilage bones in many animals. In the frog only the first 

 rudiment is cartilage bone and this is replaced in development by 

 membrane bone. 



