EXPLANATION OF PLATE I. 



Fig. 1. Male of Chimcera mongtrosa, reduced from a figure in Agassiz's Foissons Fosiile, 

 showing the opisthural filament. 



Fig. 2. Side view of embryo ray in the lophocercal stage, showing its attachment to 

 the yelk-sack by the hollow vitelline stalk which opens into an ellipsoidal 

 depressed cavity on the yelk, shown in outline. Vessels of one side of the 

 yelk only are indicated. Natural size, from a specimen taken near Wood's 

 Holl, Mass., in 1883. 



Fig. 3. Loidiocercal tail of young flounder 6 mm. long. 



Fig. 4. Lophocercal tail of youug flounder a little older than the preceding, begin- 

 ning to show a slight upbendiug of the notochord, and the first trace of the 

 permanent caudal lobe ;jc and opisthural lobe op. 



Fig. 5. Indentation appearing in tiie caudal lobe of a somewhat older flounder, per- 

 manent fin-rays being defined. 



Fig. 6. Tip of notochord still more Hexed upward than in the preceding; permanent 

 caudal and opisthural lobes somewhat more distinct. 



Fig. 7. Permanent fjc and opisthural lobes oj) now form a sharp angle where they 

 join; distinction between permanent and embryonic rays well marked. 



Fig. 8. Permanent caudal as long as opisthural lobe O}). 



Fig. 9. Cnrtilaginous supports of fin-rays are now strongly developed; the end of the 

 chorda has begun to degenerate and approximate the position which it will 

 occupy permanently as the urostyle. 



Fig. 10. The caudal lias become more rounded, the opisthure oj) is almost wholly ab- 

 sorbed and the notochord has suffered atrophy somewhat, and now presents 

 a still closer approximation to the form of the urostyle of the adult. Figs. 

 3 to 10, inclusive, after A. Agassiz. 

 1085 



