eo 



PEOr. W. K. PAEKEE OX THE 



1st stage .... 

 2nd stage .... 

 3rd stage .... 

 Hunterian adult 

 Huxley's adult 



18 

 18 

 18 

 19 

 19 



Cd. 



6 + 3 

 6 + 3 

 6 + 3 

 6+3 

 5 + 4 



So that in the 1st stage the total number in the chain is 45 ; in the 2nd and 3rd 

 stages 44 ; in the Hunterian and Huxley's adult specimens 46. But this does not 

 exhaust the variability of the axial skeleton with its inferior arches arrested or 

 developed. In the 1st stage (PL VII.) the last two cervicals have considerable styloid 

 ribs ; it has four complete rib-cinctures ; and the second dorso-sacral has ribs with a 

 half-developed sternal piece. Only those ribs that are complete below have the 

 appendage (c.a.), an oblongo-oval cartilage, not diverging backwards from the rib, but 

 parallel with it. The eight more or less developed ribs are ossifying rapidly ; these 

 are the only axial parts that are not entirely cartilaginous at present. Behind the 

 developed ribs of the second dorso-sacral there is, in this first stage, a pair of small 

 rudiments (PI. IX. fig. 4, s.r^.) which are losing their individuality already, and will 

 only appear as part of the transverse processes later on. 



There are two pairs of similar riblets in the fore part of the uro-sacral series (PI. IX. 

 fig. 4, s.r.) ; behind these, again, there are only upper transverse processes (diapophyses) ; 

 these parts are very uniform, the two pairs of uro-sacral riblets being constant in this 

 species. 



In the 2nd stage the last two cervicals have elongated or styloid ribs, and in two 

 specimens at this stage the second dorso-sacral had only small rudiments of ribs ; but 

 there were five perfect thoracic cinctures. It is seen at once that there is a vertebra 

 wanting in the dorso-sacral region in the 3rd stage as compared with the 1st (PI. IX. 

 figs. 4 & 6) : the 2nd agrees with the 3rd stage in this respect ; in both specimens of 

 the 2nd stage there is a small rib right and left on the first lumbo-sacral vertebra, and 

 also in the 3rd stage (PI. IX. fig. 6, s.i'^.) ; this shows how arbitrary is our classifica- 

 tion of the regions of the chain. The cervical vertebrae in the 1st (PI. VII.) are largest 

 and strongest in front ; behind the atlas (at.) four of these have more or less develop- 

 ment of the upper and lower spines ; there is some downward development also of the 

 atlas. The cartilage, quite unossified, is very solid, and, examined in horizontal 

 sections, it is seen that the notochord (PI. VIII. fig. 7, nc.) is submoniliform, so that 



C. Cervical ; D. Dorsal ; D.S. Dorso-sacral ; L.S. Lumbo-sacral ; S. Sacral ; U.S. Uro-sacral ; Cd. Caudal. 



