THE CHELONUN K.ULL, 



177 



Pmse 



bone, but there are large frontals. In the peri otic capsule the 

 large proOtic and the opisthotic (Cuvier's occipitale externe) 

 remain distinct bones, but the epiotic unites with the supra- 

 occipital. 



The naso-ethmoidal cartilage largely persists ; but it be- 

 comes covered above and at the sides by a large bone, which 

 meets with its fellow in the middle line, and occupies the po- 

 sition of the lachrymal, prefrontal, and nasal. The premaxilla^ 

 are small, and usually united together. There is a single 

 vomer, produced downward into a median internasal plate, 

 which expands below, and joins the palatine plate of the 

 palatine bone. 



Above the posterior and upper part of the orbit lies a post- 

 frontal, and, behind this, a squamosal is placed at the sides of 

 the periotic capsule, and above the large quadrate bone. The 

 postfrontal and spuamosal occupy the 

 upper part of the temporal region of 

 the skull. Below these, a quadrato- 

 jugal and a jugal connect the quad- 

 rate bone with the large maxilla. 



In some genera, as Chelone and 

 Chelydra, the skull possesses a sort 

 of false roof, formed by the expan- 

 sion of a median ridge, developed 

 from the parietal bones, into a broad 

 plate, which becomes suturally united 

 with the postfrontals and squamosals. 



The quadrate bone is firmly fixed 

 to the sides of the periotic region of 

 the skull, and ends below in a strong 

 condyle for the mandibles. The long 

 and broad pterygoid bones unite with 

 one another in the middle line, and 

 are firmly fixed to the base of the 

 skull, as in Plesiosauria and Croco- 

 dilia. They unite only with the up- 

 per part of the quadrate bone, as in 

 the latter reptiles. 



The palatines are firmly united Ko- 67.— The left half of the nn 



..1 .. ^ . ■ 1 1 1 • J 1 •i_-\ derside of the skull of a Turtle; 



With the pterygoids, behind, and with m\ posterior nares. 

 the vomer above and in front. They 



are prolonged downward, and develop a short palatine plate, 

 which unites with the produced and expanded lower edge of 

 the vomer, to bound the posterior nares. (Fig. 67, Fb, i^T'.) 



BS 



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