20 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



tlio ear capsule substantially as in Stage II., there being just room for 

 the eye — now, of course, increased in size — to pass between the front 

 end of it and the ethmoid. The right supraorbital becomes a little 

 more arched as the fish increases in depth. The wings of the ethmoid 

 extend out from the mid-line farther proportionally and are more flat- 

 tened antero-posteriorly. Upon the surface of these wings of the eth- 

 moid cartilage the ect-ethmoid bones, or pre-frontals, are later formed. 



irb. su^or b. s. p. iW.». eth.jlz. 



t)b. sit'orb. <lj-.f- 



'} 



crt. mk. dx. 



ms-eth. 



'•c'elfi. s. 



. . /or. olj s. 

 _ '•;■/. orb. a. 

 ...pt-pal. s. 



ha-hy. 



Fig. D. 



Oblique view of the facial cartilages of P. americanus, Stage III. Photographed from a 



model, as m the case of Fig. .-l. X circa 75. 

 For meanmg of lettering, see Abbreviations under Explanation of Plates. 



The gape has been greatly increased by the growth in length of all 

 the facial cartilages, but these have not increased in diameter propor- 

 tionately. The pterygo-palatine bars, which from the first support the 

 upper jaw, in lengthening have come to lie nearly parallel to Meckel's 

 cartilage, and their articulation with the fjuadrates is so far posterior 

 that the one of the left side alone falls within the region modelled. At 

 this stage these cartilages are in some instances so reduced in diameter 

 toward their posterior ends, as to show in cross sections only one cortilage 

 cell. A process from the left wing of the ethmoid has fused with the 



