16 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY, 



the future etlnuoid, nearly into halves. The line falls midway between 

 the two arms of the trabecuUe, where they diverge to allow space 

 for the pituitary body. In front the ethmoidal mass overlaps slightly, 

 on either side, Meckel's cartilage a little behind its points of sharpest 

 curvature. 



lu tlie flatfishes there is no distinct " tentorium," or tegmen cranii, 

 extending backward from the ethmoid to roof over the front part of the 

 brain case, as there is in the salmon. 



b. S(a(je 11. . 



Between Stages I. and II. there is an interval of six weeks and the 

 manner of differentiation of the many cartilages and projections found 



^ 



trh. su^orb. dx. 



>. 



trb. s7i'orb. .1. p. 



- trb. .tu'orb. s. a. 

 . - . ms'elfi. 

 - - ■ Vcis. eth. s. 



^ . _ ec^el/i. 



' . . crl. orb. a. 



\pl-pal. dx. 

 hn-hy. 



Kj crt. ink. 



Fig .4. 



Oblique view of the facial cartilages of P. americanus. Stage II. Pliotographed 

 from a wax model (Bonrs method) seen from a point midway between sagit- 

 tal and transverse planes and about .30' above the horizontal i>lane. X 75. 



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



in Stage II. (Fig. A and Plate 2, Fig. -10) cannot be traced here. 

 Figure 10 is a dorsal view of the facial cartilages of this stage. But, 

 as it gives a less complete view than the model of the same specimen 

 (Fig. A), I call attention to the two supraorbital bars only — the com- 

 plete one on the right (trb. su'orb. dx.), fastened to the right ethmoid 

 wing, and the two parts (a. and p.) of the left one, between which is 



