72 PROCEEDINGS OP THE AMERICAN ACADEMY 



torals is bluish, as well as the outer edge of the gill-cover. "We find 

 also in this stage the first trace of the brilliant white pigment-spots 

 which become the enamelled lines and spots so characteristic of some- 

 what older stages. In these young, they are found mainly ou the sides 

 of the gill-cover and along the line of the chorda on the anterior part 

 of the body. 



The next changes are mainly in the lengthening of the snout ; the 

 increase in length both of the lower and upper jaw (Plate III. fig. 

 23) ; the concentration of the suckers of the sucking-disk (Plate III. 

 figs. 22, 24) ; and the adoption of the general coloring of somewhat 

 older fish. The lobe of the pectoral has become specially prominent 

 (Plate III. fig. 21), and the outline of the fins is now indicated by a 

 fine milky granulation. Seen from above (Plate III. fig. 20), the 

 gill-cover is seen to leave a large circular opening leading to the gill- 

 arches, into which a current of water is constantly passing, by the 

 lateral expansion and contraction of the gill-cover; the outer ex- 

 tremity of the gill-cover covers the base of the pectorals (Plate III. 

 fig. 26). In a somewhat older stage, the snout has become more 

 elongated (Plate III. fig. 29), the suckers more concentrated, and the 

 disproportionate size of the terminal sucking-disk is reduced ; the 

 head, when seen from above, becoming slightly elongated and 

 pointed. 



In the next stage, when the young Lepidosteus is a little over three 

 weeks old (Plate III. fig. 30), the young has assumed a more fish-like 

 form ; the sucking snout is now reduced to a swelling of the extremity 

 of the elongated upper jaw ; the lower jaw has also greatly lengthened; 

 the fleshy part of the pectoral has developed out of proportion to the 

 base ; the yolk-bag has disappeared ; the gill-cover extends, when 

 pressed against the sides (Plate III. fig. 31), well up along the base 

 of the pectorals ; they are now kept in constant rapid motion, so that 

 the fleshy edge is invisible, and the vibration seems almost involun- 

 tary, producing a constant current round the opening leading into the 

 cavity of the gills. The latter are seen now to branch quite exten- 

 sively. The extremity of the tail also has the same rapid vibratile 

 motion which characterizes the pectorals. Professor Agassiz had 

 already noticed this rapid involuntary movement in the temporary 

 caudal lobe of a young Lepidosteus about eight inches in length. 



In the stages intervening between Plate III. fig. 19, and Plate III. 

 fig. 30, the young Lepidosteus frequently swim about, and become 

 readily separated from their point of attachment. In the stage of 

 Plate III. fig. 30, they remain often perfectly quiet, close to the sur- 



