STEGOCEPHALIA 



121 



by a pair of obliquely inclined bony exoccipitals, which bear the articular 

 condyles. 



The under side of the cranium is remarkable for the ilarge size of the 

 palatal vacuities, and great development of the parasphenoid, which expands 

 posteriorly into a broad, thin plate. It extends forwardly as a long, slender 

 process, and unites with the relatively large-sized vomer, the latter element 

 being paired in the earlier, unpaired in the later forms. The vomer usually 

 joins the premaxillae anteriorly, and is bounded exteriorly by the maxillae, 

 the internal nares, and front portion of the palatines. Its broad, flat- surface 

 is either edentulous or set with minute teeth, but occasionally one or two 

 powerful tusks are present in front of the narial openings, and there is a series 

 of smaller teeth bordering the anterior and lateral edges of the vomer. 



The posterior expansion of the parasphenoid unites with a trifid bone, the 

 pterygoid. The short inner arm of the latter in fact often envelops the 

 parasphenoid ; its long, forwardly directed process extends parallel with the 

 palatine and maxilla, forming the outer border of the palatal vacuity ; and its 

 short posterior branch unites with the quadrato-jugal and limits the temporal 

 vacuity. The palatines lie between the vomer and anterior extremities of the 

 pterygoid. They are bordered externally by the maxillae, as a rule, and 

 anteriorly by narial openings. They frequently bear a series of small teeth. 



The lower jaw (Fig. 227) of all Stegocephalians extends the full length of 

 the skull, and in consequence the gape of the mouth is very wide, as in Anura. 

 The mandibular ramus is composed of three pieces, of which the dentary forms 

 the tooth-bearing anterior portion, and the angular the lower portion ; behind 

 the dentary and above the angular is the articular, which bears a deep trans- 

 verse articular facette terminated behind by an elevated process. Superimposed 

 on these three pieces on the inner side of the jaw is a membrane bone called 

 the splenial (or opercular). Teeth occur in regular series, decreasing in size 

 posteriorly ; and sometimes one or two greatly enlarged teeth occur at the 

 symphysis. The union of the two rami in front was ^ 



probably ligamentous in most cases. 



The teeth ^ of the smaller Palaeozoic Stegocephalians 

 are smooth, slender, hollow, and conical (Fig. 202), and 

 implanted either directly in the supporting bone, or 

 ^ ^, attached by a cement - base. 



Very often the lower half or 

 two-thirds of the crown is ex- 

 ternally grooved or striated, in 

 which case a radial infolding 

 of the dentine extends for 



, , . , ,-,_,. nr\r,\ Lower jaw of ijj'aiuviioi-ayrus 



an equal height (.rig. 203). with simple, smooth teeth. A, 

 FIG. 203. Numerous fine dentine tubules Vi- B, Enlarged (after Credner). 



larged. A, Outer^surface! ^b] ©xteud dowuward toward the periphery, and tangen- 

 Lowerhaif'with a portion broken tially to the walls of the radial prolongations of the 



away to show the lolded dentine. >^ . r r> 



pulp cavity. In the more complicated teeth, secondary 

 and even tertiary branching of dentine tubules may occur, and at the same 

 time undulating or tortuously folded layers of cement, such as covers the 

 exterior of the tooth, may become intercalated between the bundles of dentine 



^ Creclner, H., Zur Histologie der Faltenzahne palaeozoisclier Stegocephalen (Abhancll. saclis. Ges. 

 Wiss. vol. XX.), 1893. 



