270 ZOOLOGY SECT. 



girdle-bone and representing the unossified part of the mesethmoid ; 

 and the anterior wall of each is produced into a little curved, rod- 

 like rkinal process. The whole of the primary palatoquadrate arch 

 is unossified. 



To this partly ossified chondrocranium the usual investing 

 bones are applied above and below. . Covering the roof of the 

 brain-case is a single pair of bones, the fronto-parietals (FR. PA), 

 each formed by the fusion of a frontal and a parietal, distinct 

 in the young Frog. Over the olfactory capsules are paired 

 triangular nasals (No), and applied to their ventral surfaces small 

 paired corners ( VO}. On the ventral surface of the skull is a large 

 T-shaped parasphenoid (PA. SPN), its stem underlying the basis 

 cranii, while its two arms extend outwards beneath the auditory 

 capsules. 



In the Trout, it will be remembered, the palatine and pterygoid 

 are replacing bones, formed as ossifications of the palatoquadrate 

 cartilage. In the Frog this cartilage is, as we have seen, unossified, 

 but to its ventral face two investing bones are applied, a small 

 rod-like palatine (PAL), and a three-rayed pterygoid (PTGf) having 

 an anterior arm extending forwards to the palatine, an inner arm 

 applied to the pedicle of the suspensorium, and an outer arm ex- 

 tending along the whole inner face of the suspensorium. It will 

 thus be seen that bones originally preformed in cartilage may give 

 place to investing bones, developed in corresponding situations, 

 but altogether independent of the cartilage, the latter remaining 

 unossified. 



The suspensorium, as we have seen, is strengthened on its inner 

 face by the outer arm of the pterygoid ; externally it is similarly 

 supported by a hammer-shaped investing bone, the paraquadrate, 

 often known as the squamosal (SQ). The upper jaw is formed by 

 three investing bones, the small premaxilla (PMX) in front, 

 then the long, narrow maxilla (MX), and finally the short guadrato- 

 jugal (QU. JU), which is connected posteriorly with the quad- 

 rate. 



The mandible contains a persistent Meckel's cartilage, as a sort 

 of core, outside which are formed two bones, a long angulo- 

 splenial on its inner face, and a short dentary (DNT) on the 

 outer face of its distal half. The actual distal end of Meckel's 

 cartilage is ossified as a small replacing bone, the mento-mecJcelian 

 (M. MCK), not represented in Fishes. 



The hyoid apparatus consists of a shield-shaped plate of car- 

 tilage, the "body of the hyoid (b. hy), produced at its anterior angles 

 into slender rods, the anterior cornua (a. c. hy), which curve upwards 

 and are fused with the auditory capsules, and at its posterior angles 

 into partly ossified rods, the posterior cornua (p. c. hy), which 

 extend backwards, embracing the glottis. 



Two other cranial structures remain to be noticed. External 



