OCR Text |
Show 1897.] SKELETON OF PELODYTES PUNCTATUS. 589 STAGE 6. Distance from snout to root of tail, 19 mm. Length of tail, 2'5 mm. Length of hind limb, extended, 27 mm. Length of fore limb, extended, 11 mm. (Plate X X X V . fig. 6.) l At this stage the branchial arches are on the verge of dissolution, and it requires the very greatest care in dissection to avoid losing the relations of the dismembered parts. The whole aspect of the hyobranchial skeleton has changed. In the middle is a faint Y-shaped mark representing the lines of junction of the two hypobranchial plates and the basihyal. The mesial edges of the ceratohyals can still be identified, but they are wider apart than before. The hyoglossal notch is reduced in size by the addition of cartilage (pa., fig. 6) to the front half of the antero-internal free edges of the ceratohyals. Although this new cartilage is confluent with that of the ceratohyal, what was previously the edge of the latter is still well-marked, thus showing that the new process is not due to an outgrowth of the original hyoidean cartilage. These new cartilages represent the anterior processes of the hyoid apparatus of the Frog (pa., fig. 11) which develop late and about this stage. In fact, the figure which Gaupp gives of the hyoid of the metamorphosing tadpole of Rana fusca with tail reduced to 5 m m . (5. Taf. 19. fig. 8) agrees tolerably well with fig. 6 of Pelodytes. Gaupp applies the name " processus anterior" indifferently to the most anterior point of the original ceratohyal (" hyale ") and to the later developed cartilage which constitutes the foremost part of the adult hyobranchial skeleton ; but, seeing that the latter has a morphological significance different from the former, and is, moreover, absent in such genera as Alytes, Dlsco-glossus, and Bomblnator possessing the former, I have elected to restrict the application of the term to the latter. Parker's terminology does not help matters, but rather tends to confusion, for he marks these anterior processes of the adult hyoid as parts of the ceratohyals (12. pl. 13. fig. 10), as hypohyals (pl. 5. fig. 4), as epihyals (pl. 15. fig. 14), and as extrahyals (pl. 43. fig. 6), without offering any explanation of the want of uniformity. The backward slope of the ceratohyals is n o w very strongly marked and the posterior parts are becoming reduced in width. The terminal articular surface is quite small compared with its previous extent, and the articulation will shortly give place to a rigid connection with the auditory cartilage. The thyrohyals continue to grow, but their posterior extremities are still capped with unabsorbed ceratobranchial cartilage. The outlines of the branchial arches are very indistinct, the softness of the cartilage making it difficult to distinguish them from the surrounding connective tissue and pharyngeal mucous membrane. The tri-radiate cartilage at the proximal end of the second and third branchial arches appears to be constant in its occurrence, and is 1 These measurements are those of the figured specimen, which is slightly smaller than the average; but if the above figures be multiplied by \l the measurements will be seen to be intermediate between those of the specimens representing Stages 5 and 7. PROC ZOOL. Soc-1897, No. XXXIX. 39 |