xiii PHYLUM CHORDATA 251 



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

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

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

 in the young Frog. Over the olfactory capsules are paired tri- 

 angular nasals (NA), and applied to their ventral surfaces small 

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

 T-shaped parasphenoid {PA. SPH), 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 cartilage bones, formed as ossifications of the palato-quadrate 

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

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

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

 be seen that, as we ascend the animal series, bones originally 

 preformed in cartilage may give place to membrane 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 membrane bone, the squamosal (SQ). 

 The upper jaw is formed by three membrane bones, the small pre- 

 maxilla (PMX) in front, then the long, narrow maxilla (MX), and 

 finally the short quadrato-jugal (QU. JU), which is connected 

 posteriorly with the quadrate. 



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

 of core, outside which are formed two membrane-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 -cartilage bone not represented in 

 Fishes, the mento-meckelian (M. MCK). 



The liyoid 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 

 to the squamosal is a ring of cartilage, the annulus tympanicus 

 (Fig. 882, an. tymp), which supports the tympanic membrane as 

 the frame of a tambourine supports the parchment. Inserted into 

 the fenestra ovalis is a nodule of cartilage, the stapes (stp), to 

 which is attached the inner end of a small hammer-shaped struc- 

 ture, the columella (COL), the handle of which is ossified, while its 



