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Show 592 DR. w. G. R I D E W O O D O N T H E H Y O B R A N C H I A L [May 18, slope inwards and forwards (fig. 6), then directly 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 width at their posterior extremities, and, although they cannot yet be said to be ossified, the shaft is slightly more opaque than the cartilage 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 X X X V . 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 anterolateral 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 Erog 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 (cf. figs. 7 and 8). The postero-lateral process has developed an irregular plate of cartilage at its extremity, and, since the extremity of the anterolateral process has grown backwards as wrell 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 iu length and underlie the anterior parts of the thyrohyals. The free 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 n o w present, and are inseparable from the dentary membrane-bones. They are not, as might be supposed, |
