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- 
trb. arc. eth. m. ec’eth. 
See STE eT 
j fOr. Olf Ss 
1 
ham. eth oe : oo 
oe pl-pal. s. 
ert. orb. a. 
i 
we at ee ae 
_. pt-pal. dz. 
oe | for. olf. dz. 
“ hebesh. dz. 
ert. mk. 
Exe. we: 
Oblique view of the facial cartilages of P. americanus. Stage IV. Viewed from the same 
direction as in Figs. A. and B. X 70. 
For meaning of lettering, 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 trabeculee cranii at the left (ventral) shown in Figures C’ and D 
has therefore in the growth of the cartilage been left to accommodate 
the oblique eye muscles, just as the olfactory foramina in the ethmoid 
were left because of the presence of the olfactory nerves. 
The now ventrally projecting right ect-ethmoid partially hides in a 
1 This 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 night of the dotted line, arc. eth. m. 
