The Emhryology of Heterodontus japonicus 761 



the palatoquadrate cartilages covered with mucous membrane) is readily seen between the 

 olfactory region anteriorly, and what appears to be a breathing valve^ posteriorly. The 

 small pit represented in the center of the figure presumably leads into Rathke's pouch. 



Text-figure 67, like the preceding, represents an upper jaw. It is the first drawing, of 

 this series, portraying teeth. Presumably, this drawing was made from a recently hatched 

 specimen. (For similar teeth of a recently hatched H. phillipi, observe Text-figure 13, 

 page 672). The anterior teeth represented in Text-figure 67 are larger than the posterior 

 teeth (contrary to the condition in the adult) and each anterior tooth possesses five cusps. 

 Posteriorly, the number of cusps grades from five to none. More distinctly than in Text' 

 figure 66, the breathing valve appears divided into two main portions, anterior and pos- 

 terior respectively. The central third of the anterior portion is subdivided into a large 

 number of short lobules. The filamentous portions of the breathing valve bear an irregular 

 fringe of lobules. 



Text'figure 68 portrays what is apparently the same stage in a lower jaw. This 

 drawing should be compared with Text'figure 13, showing the teeth of a recently 

 hatched specimen of H. phillipi. There is no sharp division between anterior (cuspidate) 

 and posterior (grinding) teeth, and the total number of transverse rows is less than in the 

 adult. The anterior two'thirds of the teeth are typically five-cusped. The extreme 

 posterior teeth are almost or quite lacking in cusps. In the intermediate region, the num- 

 ber of cusps is usually four. As in the upper jaw, the extreme posterior teeth, which lack 

 cusps, are smaller than the largest anterior teeth. In the absence of any drawing showing 

 the teeth in an earlier stage, it seems probable that most of the anterior teeth possess five 

 cusps from the beginning — for it is known that Dean had a fairly close series of stages from 

 which to select specimens for drawings. In the figure under consideration there is a row 

 of unusually large oral denticles situated close to the inner margin of the jaw. Between 

 these denticles and the teeth drawn in broad view, one may see the serrated edges of an 

 inconspicuous inner row of teeth. 



Text'figure 69 represents the roof of the mouth cavity of the 280'mm. female Hetew 

 dontus japonicus portrayed in Text'figure 65, page 756. Text'figure 69 depicts faithfully 

 not only the form but the precise number and arrangement of the teeth in this specimen. 

 There are three longitudinal rows, with an extra tooth at the extreme posterior end 

 making a transverse row of four. The anterior teeth are still typically five'cusped. Two 

 transverse rows of the most posterior teeth lack cusps, but each of these teeth bears 

 a prominent longitudinal ridge. The posterior ridged teeth are much larger (especially 

 longer) than the anterior teeth. The transition between anterior (cuspidate) and posterior 

 (grinding) teeth is more abrupt than it is in earlier stages. The middle portion of the 

 anterior division of the breathing valve consists of a compact group of long finger'like 

 lobules — which may be seen more clearly in the specimen than in the drawing. 



' I have found this problematical breathing valve not only in the 78-mm. embryo of Heterodontus japonicus, but also in the 280-mm. 

 specimen of the same species and in a young 368-mm. H. quoyi. I have had no opportunity to observe it in the living fish, hence cannot 

 state positively what is its function; but its position and structure suggest strongly that it is a breathing valve. 



