OF ARTS AND SCIENCES. 71 



and the pectorals have increased in size. The whole body is more 

 thickly covered by black pigment cells : they are most numerous 

 near the region of the head, along the dorsal line, and over the intes- 

 tines and the upper part of the yolk-bag, making a strong contrast to 

 its yellowish-gray color underneath. The curvature of the tail has 

 increased, and four patches of pigment spots have appeared (Plate II. 

 fig. 14). These are the first traces of the permanent fins, — the dorsal, 

 the caudal, and anal, — the last spot on the dorsal side of the tail 

 forming the spot of the temporary caudal lobe. The gill-arches in 

 this stage (Plate II. fig. 16) are thin, short, club-shaped appendages, 

 seen on certain portions behind the gill-covers. 



The great development taken by the sucking-disk is best shown in 

 a view from the lower side. The cavity of the mouth is seen (Plate 

 II. fig. 17) to occupy the greater part of the lower side, between the 

 sucker and the base of the yolk-bag, where the heart is placed. Five 

 or six days afterwards, the cavity of the mouth has become reduced to 

 a small (Plate II. fig. 18) trapezoidal opening; the gill-covers nearly 

 meet on the lower side, near the median line ; and in this stage the 

 individual suckers of the sucking-disk are capable of great expansion, 

 projecting, after the young fish has remained attached for a time, far 

 beyond the general outline of the anterior part of the head. When 

 seen in profile (Plate III. fig. 19), the outline of the head has assumed 

 a most peculiar appearance, having certainly no resemblance whatever 

 to the shape of the head in the adult. The yolk-bag has become much 

 reduced in size ; the pectorals have now assumed the characteristic 

 Crossopterygian features ; the eye is quite prominent ; the lower jaw 

 projects slightly beyond the former level of the mouth cavity ; and 

 there is also a short upper jaw, terminating in the huge swollen snout, 

 covered with warts, forming the powerful sucking-disk. The gill- 

 cover is still more elongated than in previous stages, and there are 

 short branches to the primary gill-arches. The whole body is now 

 thickly covered with black pigment cells. These cells are, however, 

 still most numerous above the eye, toward the dorsal region, along 

 the upper part of the yolk-bag, where they are so closely packed as 

 to form a dense black band, which is continued on the lower side of 

 the chorda to the extremity of the tail. The black spots of the primary 

 fins have expanded to form definitely shaped patches indicating the 

 growth of the permanent fins, of which they are the rudiments. The 

 extremity of the tail has a more marked heterocercal character, and 

 both above and below the tail carries a large white patch completely 

 surrounded by the darker pigment-cells. The fleshy lobe of the pec- 



