26 THE BONES AND JOINTS. 



obtuse angle, the third cartilaginous piece, the extra- 

 stapedial (Fig-. 12 a"}. It is attached to the tympanic 

 membrane, and by its upper portion is fastened to the 

 cartilaginous tympanic ring by a smaller piece, the supra- 

 stapedial. 



3. The tympanic ring (cumulus tympanicus) is an annular cartila- 

 ginous frame ; or more exactly, has the shape of a short, truncated 

 cone, as it narrows towards the middle line : it is attached to the 

 squamosal bone. (See Organ of hearing.) 



3. The parasphenoid, os sphenoideum, Cuvier (Figs, n and 



Cuvier, I.e., p. 388, PI. XXIV, d. Duges, n. 8. Meckel, Theil des 

 Grundbeins. Parker and Bettany, Z. c., parasphenoid. 



A large portion of the base of the cranium is taken up by this 

 cruciform bone. Of the two longitudinal median processes, the 

 posterior is by far the shorter, and lies in front of and partly 

 below the cartilaginous os occipitale basilare. The anterior longer 

 longitudinal arm closes in the greater part of the cranium from 

 below, and articulates by its outer edges with that part of the 

 prootic bones often described as the alae magnae, and also with the 

 cartilage lying in front, which forms the greater part of the lateral 

 walls of the cranium. The anterior extremity of the bone articulates 

 with the palatine bones. The transverse arms lie on the under 

 surface of the exoccipitals and of the prootics. 



The greater width of the cranium in R. temporaria is associated 

 with the greater relative width of the anterior arm of this bone. 



4. The fronto-parietal bones, ossa fronto-parietalia, Cuvier 

 (Figs. TO and i^-fp). 



Cuvier, I.e., p. 387, PI. XXIV, c.c. Duges, n. i. Parker and Bettany, 



I.e., fronto-parietal. 



These are a pair of somewhat long, flat bones, which form the 

 principal part of the upper wall or roof of the cranium, and cover 

 in superiorly the cartilaginous cranium, which is here, in great 

 part, persistent. They are united in the middle line by the sagittal 

 suture ; posteriorly they articulate with the exoccipital and prootic 

 bones ; anteriorly with the sphenethmoid, which they overlap like 

 tiles. The outer margin of each bone is bent somewhat down- 

 wards (Fig. \6fp), and between it and the parasphenoid there is a 

 space in the wall of the cranium which is closed in by cartilage and 

 connective tissue only. 



