592 DB. W. G. BIDEWOOD ON THE HTOBBAJSCHIAL [May 18, 



slope inwards and forwards (fig. 6), then directl}' inwards (fig. 7), 

 and afterwards inwards and backwards (fig. 8). They are nearly 

 in contact in the middle line, and it requires but little additional 

 growth to close the hyoglossal sinus and convert it into a foramen. 

 The liberated part of the ceratohyal expands slightly behind its 

 anterior pointed extremity and then diminishes again into a curved 

 rod, the posterior end of which is attached to the back of the 

 auditory capsule. The thyrohyals have increased in wddth at their 

 posterior extremities, and, although they cannot yet be said to be 

 ossified, the shaft is slightly more opaque than the cartUage of 

 the epiphysis behind and the basal plate in front. The ventral 

 ossification is disposed as in the adult, but its postero-lateral rays 

 are very short. The anterior parts of the mandibular cartilage 

 have not yet ossified to form the mentomeckelian bones. 



Stage 9. Adult. Distance from snout to cloaca, 37 mm. Length 

 of hind limb, extended, 62 mm. Length of fore limb, extended, 

 26 mm. (Plate XXXV. fig. 9.) 



Since the structure of the adult hyobranchial skeleton has already 

 been described in the opening paragraphs, it will be sufficient to 

 confine attention here to those changes which have occurred since 

 Stage 8. The differences in appearance are due chiefly to the 

 addition of cartilage to the periphery. This marginal addition in 

 the case of the anterior part of the hyoid arch and the antero- 

 lateral process has been so great as to completely enclose the first 

 of the three lateral sinuses, forming a lateral foramen. The 

 anterior processes of the hyoid have grown inwards in front of 

 the hyoglossal sinus so as to touch one another, or even to overlap. 

 It is only by analogy with the Frog that the term "processus 

 anteriores " is applied to these enclosing cartilages ; it is highly 

 probable that the most anterior points of the hyobranchial skeleton 

 belong to the original hyoidean cartilage and not to these processes 

 ^/. figs. 7 and 8). 



The postero-lateral process has developed an irregular plate of 

 cartilage at its extremity, and, since the extremity of the antero- 

 lateral process has grown backwards as well as forwards, the 

 middle of the three lateral sinuses exhibits a tendency to become 

 enclosed and converted into a foramen like that situated in front 

 of it. The thyrohyals are now completely ossified and are more 

 parallel than before. The postero-lateral rays of the ventral splint- 

 bone have increased in length and underlie the anterior parts of 

 the thyrohyals. The fi'ee parts of the hyoidean cornua are 

 variable in shape, but the anterior end is always flat and lamellar, 

 while the posterior has the form of a curved rod of cartilage. A 

 comparison of this stage with the two preceding shows that the 

 part of the hyoidean cornu missing is quite inconsiderable, and is 

 to be estimated by the mean distance from the point marked h in 

 fig. 9 to the anterior extremity of the detached part of the arch. 

 Mentomeckelian bones are now present, and are inseparable from 

 the dentary membrane-bones. They are not, as might be supposed, 



