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Show 1897.] SKELETON Ob1 PELODYTES PUNCTATUS. 587 criticism of Parker's paper contained in the footnote which he appends. The mode of development of the thyrohyals of Pe'-odgtes by the formation of thyroid foramina and their subsequent disruption is calculated to shake one's faith in the recognition in the thyrohyals of Anura of late and rapidly developing fifth ceratobranchials as suggested by Meckel (8. p. 240) and Cuvier (3. p. 397); and, although the arguments which I have previously adduced in favour of this view (16. p. 112) still hold good, I must confess to entertaining a suspicion that, after all, these bones belong to the hypobranchial rather than to the ceratobranchial part of the visceral skeleton. The mandible no longer exhibits the sharp angulation of its earlier stages. It is considerably longer than before, but the four constituents can still be clearly recognized. The membrane on the internal or mesial surface of the ramus is much thickened, and it is this which, when ossified, becomes the angulosplenial bone. There is as yet no trace of a dentary. STAGE 4. Distance from snout to root of tail, 20 mm. Length of tail, 23 mm. Length of hind limb, extended, 28 mm. Length of fore limb, extended, 12 mm. (Plate X X X V . fig. 4.) The whole aspect of the hyobranchial skeleton is beginning to change. The ceratohyals are now massive cartilages at their maximum of development, and they slope more posteriorly than before. The hyoglossal notch is both broader and deeper than in Stage 3. The triangular space on either side of the basihyal has now become filled up, not by encroachment of the surrounding cartilages, but by the differentiation of new cartilage : the outlines of the three cartilages bounding the former space are still very clearly marked. There are unmistakable signs that the branchial skeleton is past its prime. The arches exhibit a condition of incipient collapse, but are not very much smaller than in the preceding stage. The spicula have almost disappeared. There is a stump remaining of that of the second ceratobranchial, and the developing thyrohyal is still tipped with a remnant of that cartilage which probably represents the fourth spicule, but the first and third spicules have been completeby absorbed. The thyroid foramen ((/"., fig. 4) is now a distinct perforation of the cartilage, and the thyrohyal is beginning to assume shape. Lying at the bottom of the laryngeal sinus, between tbe two thyrohyals, is a tract of soft cartilage of crescentic form which only in this stage acquires a definite outline. The sinus in which it lies has been increasing in size from the very first stage, and, as will be seen by referring to the figures 5-8, continues on the increase. In the first three stages the loose cartilage occupies a larger proportion of the space, but the tissue is of such an ill-defined character that it is a matter of personal opinion whether to regard it as cartilage at all; and, on account of its undifferentiated nature, it is impossible to recognize its posterior limit. It is therefore omitted in the first three figures. The cartilage is in |
