A 74 MM. POLYODON. 2O3 



of the rostrum. His tables show a steady increase in the relative 

 length of the rostrum in passing from larger to smaller individuals. 

 My own measurements reveal the presence of somewhat striking 

 individual variations, but it will be seen, nevertheless, on ex- 

 amining the table that in the first ten the length of the snout is, 

 on the whole, gradually increasing while with the latter ten the 

 reverse is the case. The smallest fish measured by Professor 

 Stockard was approximately 600 mm. in length and had a ros- 

 trum represented by the fraction .333. The largest of the small 

 fish measured for the table presented herewith was 200 mm. long 

 with a rostrum represented by .390. The available data, there- 

 fore, seem to indicate that the rostrum attains its maximum 



FIG. 3. Dorsal view of the same fish. 



development at a time when the fish is from 200 mm. to 600 mm. 

 in length. The biology of the species is too little known to 

 enable one to say whether or not there is any functional signifi- 

 cance :n such a development at this time. 



In adult Polyodon the fins show a marked angularity. In the 

 embryo they are rounded and much less distinctive. This dif- 

 ference will become evident on comparing the figures accompany- 

 ing Professor Stockard 's article with those presented here. The 

 position of the various fins does not seem to differ greatly between 

 the small and the large fish. 



The tail of the embryo is rather strikingly unlike that of the 

 adult. It is much more obviously heterocercal and the dorsal 

 part is somewhat lobed as shown indistinctly in Fig. I. The 

 small terminal lobe, which is not found in the adult, seems to 

 be either without rays or with rays only feebly developed. It is 

 a fleshy mass with apparently no tendency to become elongated 

 into a slender filament. It is not clear from the material at 



