THE SKULL. 23 



A. The Cranium. 



The cranium of the frog- is a prismatic tube, wide behind, narrow 

 in front, and formed in great part of cartilage (Figs. 15 and 17). Our 

 indigenous species are characteristically distinguished from one 

 another by peculiarities in the form of the cranium. In R. escideufa 

 it is long and narrow, in R. temporaria short and wide. The superior 

 surface in the former is markedly concave, while in the latter it is 

 flat, and in R. oxyrhinus arched. These differences are readily 

 recognised in the living animal. 



The Bones of the Cranium. 



1. The exoccipital bones, ossa occ'qntaUa laferalia, Cuvier 

 (Figs. 10, II, 12, 14, 16 0). 



Cuvier, I. c, p. 387, PI. XXIV, bb.— Dugès, I. c, n. 14. — Parker and Bet- 

 tany, I. c, p. 166, exoccipitals. 



These paired bones form the hinder part of the cranium ; they 

 bound the foramen magnum, and articulate with the vertebral column. 

 They are imbedded in the cartila- Fig. 12. 



ginous matrix of the skull, and are 

 separated above by an unossified 

 part of this matrix {occipitale 

 superius, Dugès), which represents 

 the tabular portion of human 

 anatomy: below they are separated ^^"" ""^ ^'«"" '''.''^"''^' ^'f " ^^•'^"' ^^'^'^^' 



_*' _ . . . twice natural size. 



by a similar part {occipitale basi- „ stapes. 

 lare, Dugès) which represents the "'„ CoiumeUa anns. 



> o I r d" Bxtrastapedial. 



body of the occij)ital bone. They c condyles of exoccipitals. 



„ , • ^ h Stylo-hyoid. 



therefore properly represent only „ Exoccipitals. 



the condylar portions {partes con- J, p^o^^'to which the jaw is attached. 



di/loideae) of the human occipital t Squamosal. 



bone. Each possesses a cartilaginous articular head, for articulation 



with the first vertebra : these converge below, and surround the 



lower half of the circumference of i\\e foramen magnnm. This latter 



has, in R. esculenta, a transversely oval outline ; in R. temporaria, a. 



somewhat heart-shaped outline, with the apex directed upwards : 



in accordance with this the whole bone is wider than high in the 



first species ; and in the latter it is higher than it is wide. From 



the upper and outer border of the foramen magnum on each side 



