286 THE COD FAMILY. 



inferiorly. The ventral fins are large, nearly a quarter the 

 length of the body, and they are deeply pigmented throughout. 



At 6 mm. (7th May) the marginal fin is still continuous, 

 with embryonic rays, while true rays are developing in the tail 

 above and below the notochord, which is slightly bent upwards 

 and buttressed by a superior and inferior thickening (epiural 

 and hypural). The pectoral fins are larger, and the ventrals 

 are still of great length and deeply pigmented throughout 

 (Plate XII, fig. 3). The top of the head and the dorsum over 

 the pectoral fins have black chromatophores, and touches of the 

 same pigment are present along the posterior region of the 

 dorsum, and on the middle of the body occupying a corre- 

 sponding line to the four or five chromatophores which alone 

 remain on the ventral edge in front of the tail. Thus changes 

 have already begun in the pigmentation. The abdomen is 

 slightly silvery. 



When 8 mm. long in spirit (23rd May) only one black 

 chromatophore remains on the ventral edge in front of the tail, 

 while the lateral pigment has increased, and the dorsal has 

 extended forward to amalgamate with that above the pectorals. 

 The abdomen and the sides — to within a short distance of the tail 

 — are silvery. True rays exist in the dorsal and anal fins, but 

 each is joined to the tail by the larval fin. The ventral fins 

 are still very long. A little later (at 9'9 mm.), the black 

 pigment of the ventral fins is confined to the distal region. 



At 11 mm. (Plate XII, fig. 4, the example being 10 mm.) 

 the pectorals still remain pale, though proportionally larger and 

 with true rays. The ventrals have now pigment only on 

 the tip, rather less than half the fins being black. The 

 pigment has disappeared from the ventral edge of the body, 

 and has increased along the dorsum, on the snout and on the 

 cheeks. The general silvery hue is more conspicuous. No 

 barbels are yet present. Such forms are known as " mackerel- 

 midges." The black pigment has largely increased on the 

 dorsum and sides at 14 mm., and also the silvery sheen. 

 Touches of black likewise occur on the sides of the mandible 

 ventrally. The history of the discovery of these midges and 

 the determination of their true affinities is worth narrating. 



