26 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The external opening of the left nasal pit is about 30° higher in 

 Stage IV. (Fig. 5) than in Stage II. (Fig. 4). 



The superior oblique muscles of the eyes have their origins at or near 

 the junction of the median arch with the mes-ethmoid. The inferior 

 oblique of the right eye is attached to the ethmoid on the dorsal (mor- 

 phologically left) side of this median arch and that of the left eye im- 



Irb. arc. elh. m. ec''eih. " 



hnm. till 



pt pal. dx. 



rl. orb. n. 



pt-pal. <!z. 



for. olf. dx. 

 ■ 'ec''eth, dx. 



crt. mk 



ba-hy. 



Fig. D. 



Oblique view of the facial cartilages of P. americanus. Stage IV. Viewed from tlie same 



direction as in Figs. .1. and B. X 70. 

 For meaning of lettermg, see Abbreviations under Explanation of Plates. 



mediately behind that of the right. The large passage ^ between the 

 ethmoid in front, the median arch at the right (morphologically dorsal), 

 and the trabeculsc cranii at the left (ventral) shown in Figures C and D 

 has therefore in the growth of the cartilage been loft to accommodate 

 the oblique eye muscles, just as the olfactory foramina in the ethmoid 

 were left because of the presence of the olfaQtory nerves. 



The now ventrally projecting right ect-ethmoid partially hides in a 



1 Tliis passage is seen in Figure C" directly above the pointed end of the probe 

 inserted through the right olfactory foramen ; it is indicated in Figure D by a 

 triangular area at the right of the dotted line, arc. eth. m. 



