STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 757 



No traces of the pelvic fins have yet become developed, _ 



The positions of the permanent dorsal, anal, and caudal fins, 

 as pointed out by Agassiz, are now indicated by a deposit of 

 pigment in the embryonic fin. 



In an embryo on the sixth day after hatching, of about 15 

 millims. in length, of which we have also given a figure (Plate 

 34, fig. 14), the following fresh features deserve special notice. 



In the region of the head there is a considerable elongation 

 of the pre-oral part, forming a short snout, at the end of which 

 is placed the suctorial disc. At the sides of the snout are placed 

 the nasal pits, which have become somewhat elongated anteriorly. 



The mouth has lost its open rhomboidal shape, and has 

 become greatly narrowed in an antero-posterior direction, so 

 that its opening is reduced to a slit. The mandibles and max- 

 illary processes are nearly parallel, though both of them are 

 very much shorter than in the adult. The operculum is now a 

 very large flap, and has extended so far backwards as to cover 

 the insertion of the pectoral fin. The two opercular folds nearly 

 meet ventrally. 



The yolk-sack is still more reduced in size, one important 

 consequence of which is that the pectoral fins (pc.f.} appear to 

 spring out more or less horizontally from the sides of the body, 

 and at the same time their primitive line of attachment to the 

 body becomes transformed from a longitudinal to a more or less 

 transverse one. 



The first traces of the pelvic fins are now visible as slight 

 longitudinal projections near the hinder end of the yolk-sack 



The pigmentation marking the regions of the permanent fins 

 has become more pronounced, and it is to be specially noted 

 that the ventral part of the caudal fin (the permanent caudal) is 

 considerably more prominent than the dorsal fin opposite to it. 



The next changes, as Agassiz points out, " are mainly in the 

 lengthening of the snout ; the increase in length both of the 

 lower and upper jaw ; the concentration of the sucker of the 

 sucking disc ; and the adoption of the general colouring of 

 somewhat older Fish. The lobe of the pectoral has become 

 specially prominent, and the outline of the fins is now indicated 

 by a fine milky granulation. Seen from above, the gill-cover is 



