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Show 588 DR. W. G. RIDEWOOD ON THE HYOBRANCHIAL [May 18, the later stages gradually absorbed from behind, and disappears altogether at about Stage 8. The mandible has elongated considerably, and the articular ends have lost the sharp bend which they possessed during the first three stages. The four segments of the mandible are still just recognizable. The angulosplenial and dentary ossifications of the perichondrium (as. and d., tig. 4) are both present, but they are very fibrous and perfectly flexible. STAGE 5. Distance from snout to root of tail, 20 mm. Length of tail, 8 mm. Length of hind limb, extended, 30 mm. Length of fore limb, extended, 12 mm. (Plate X X X V . fig. 5.) The branchial skeleton is in an advanced stage of reduction, and its maximum width is considerably smaller than the width across the hyoidean cornua, which has not been the case previously. The specimens of this stage which I have examined confirm my previous observation with regard to the branchial skeleton of Plpa (16. p. 105) that there is a marked shrinkage of the cartilage, and consequently of the clefts, before the latter are broken open by the absorption of the cartilage. The three clefts in the branchial plate are, in this stage, still enclosed by the cartdage, but they are not more than one-half of the length of the clefts in Stage 4. There are marked indications that, as in Plpa (16. p. 106), the first commissural cartilage to yield is that joining the first and second ceratobranchials. The thyrohyals have enlarged considerably, and are composed of a firm hyaline cartilage in sharp contrast with the softer, wdiiter, and more opaque cartilage of the parts of the branchial skeleton undergoing absorption. The thyroid foramen is also larger than before, and there now remains but a thin neck of cartilage between it and the first branchial cleft; so that the second and third ceratobranchials appear to be connected with the hypobranchial parts of the skeleton by a common isthmus, while the first and fourth ceratobranchials are attached more directly. These facts were to be observed in Stage 4, but are more obvious here in consequence of the branchial skeleton being flatter. There is still recognizable on the anterior edge of the second ceratobranchial the cartilaginous promontory which was in the earlier stages bound to the first ceratobranchial by fibrous tissue. The ceratohyals are directed more backwardly than in Stage 4, the outlines of their inner edges are becoming obscured, and, curiously enough, more removed from one another-a fact which is emphasized still more in Stage 6. The basihyal is still recognizable, but its contour is becoming fainter. The hyoglossal notch or sinus is now at its maximum size. The mandible is of large size and is distinctly U-shaped. There are now only two segments to the mandible, the more median parts (the lower labial cartilages of Parker) having fused with the more lateral parts. The angulosplenial and dentary ossifications are much larger than before. |
