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Show 586 DR. W. G. RIDEWOOD ON THE HYOBRANCHIAL [May 18, beginning. The hyoidean constituents are slightly larger than before, and the branchial are smaller. There is still a triangular space on either side of the basihyal: the spicula of the ceratobranchials are shortening up. The most interesting feature to be noted in this stage is the fenestration of the hypobranchial plate in those regions situated immediately over the thyroid bodies. The cartilage in the area marked tf. in fig. 3 is extremely thin, and becomes converted into a distinct perforation in Stage 4. The postero-internal margin (t) of the incipient foramen soon becomes rod-like. It enlarges rapidly, and when, later, the absorption of the external boundary causes the thyroid foramen to open laterally, it becomes recognizable as the thyrohyal (t, figs. 4-7). W h e n recounting recently the mode of development of the hyobranchial skeleton of Plpa, it was with considerable hesitation that I described (16. p. 106 et seq.) the thyrohyals as persistent parts of the hypobranchial cartilage left by the perforation of the plate and the subsequent conversion of the foramina into deep sinuses. This view of the mode of development was so opposed to all preconceived notions that it seemed little short of heresy to give expression to it, and it was only after repeated examinations of the specimens that I could bring myself to publish the observation. The present discovery of a similar origin of the thyrohyals in a less aberrant type of Batrachian not only confirms m y former view, but opens up the broader question whether, after all, this may not be their normal mode of development in Anura. A glance at figs. 3, 4, and 5 shows that the thyrohyals are terminated by those processes wdiich, following Gaupp, I interpret as the spicula of the fourth branchial arch. These might easily be mistaken for the free ends of newly outgrowing thyrohyals, although, as a matter of fact, they are absorbed with the ceratobranchial cartilages, leaving the extremities of the true thyrohyals freely exposed. And thus become reconciled the apparently conflicting views of Saint-Ange (17. pp. 410 and 417), who considers the thyrohyals as the persistent spicules of the fourth branchial arch, and Cuvier (3. p. 397), Bathke (14. p. 39), Duges (4. pp. 99, 101, 102), Stannius (19. p. 65), Gtitte (6. p. 332), Parker (11. pp. 164, 170, 171, 185, and 12. p. 259), and Gaupp (5. pp. 422 and 433), wdio regard them as outgrowths of the hypobranchial plate situated behind and to the mesial side of the fourth branchial arch. The alternative view that the thyrohyals of Anura are persistent ceratobranchials has been supported by Beichert (15. pp. 59 and 258), by OwTen (10. p. 90), by Parker, who in his textbook (13. p. 173) unfortunately departs from the views expressed in his memoirs (I. c.) and states that the thyrohyals are the fourth ceratobranchials, and by Walter (21) and Cope (2. p. 234), who accept Parker's determination without reserve. Stohr (20. p. 84), also, ventures upon a positive statement in favour of the thyrohyal being the ventral or proximal end of the fourth branchial arch ; and that he does not mean by this the hypobranchial constituent of the arch is evident from his |