588 mi. w. G. RiuEwooD on the HYOBRANciiiAL [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 o£ the mandible are still just 

 recognizable. The angulosplenial and dentary ossifications of the 

 perichondrium (as. and d., fig. 4) are both present, but they are 

 very fibrous and perfectly flexible. 



Stage 5. Distance from snout to root of tail, 20 mni. Length of 

 tail, 8 mm. Length of hind limb, extended, 30 mm. Length of fore 

 limb, extended, 12 mm. (Plate XXXV. 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 Pipa (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 cartilage, but they are 

 not more than one-half of the length of the clefts in Stage 4. 

 There are marked indications that, as in Pipa (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, whiter, and more opaque cartilage of the pai'ts 

 of the branchial skeleton undergoing absorption. The thyroid 

 foramen is also larger thao 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 

 behig 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 recog- 

 nizable, 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 no\v 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 ossifica- 

 tions are much larger than before. 



