756 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



strong current being made by the rapid movement of the pectorals, 

 against the base of which the extremity of the gill-cover is closely 

 pressed. The large yolk-bag is opaque, of a bluish-gray colour. 

 The body of the young Lepidostens is quite colourless and trans- 

 parent. The embryonic fin is narrow, the dorsal part commencing 

 above the posterior end of the yolk-bag ; the tail is slightly 

 rounded, the anal opening nearer the extremity of the tail than 

 the bag. The intestine is narrow, and the embryonic fin extend- 

 ing from the vent to the yolk-bag is quite narrow. In a some- 

 what more advanced stage, hatched a few hours earlier, the 

 upper edge of the yolk-bag is covered with black pigment cells, 

 and minute black pigment cells appear on the surface of the 

 alimentary canal. There are no traces of embryonic fin-rays 

 either in this stage or the one preceding ; the structure of the 

 embryonic fin is as in bony Fishes previous to the appearance 

 of these embryonic fin-rays finely granular. Seen in profile, 

 the yolk-bag is ovoid ; as seen from above, it is flattened, rect- 

 angular in front, with rounded corners, tapering to a rounded 

 point towards the posterior extremity, with re-entering sides." 



We have figured an embryo of 1 1 millims. in length, shortly 

 after hatching (Plate 34, fig. 12), the most important characters 

 of which are as follows : The yolk-sack, which has now become 

 much reduced, forms an appendage attached to the ventral 

 surface of the body, and has a very elongated form as compared 

 with its shape just before hatching. The mouth, as also noticed 

 by Agassiz, has a very open form. It is (Plate 34, fig. 13, m.} 

 more or less rhomboidal, and is bounded behind by the mandi- 

 bular arch (?;/.) and laterally by the superior maxillary processes 

 (s. mx). In front of the mouth is placed the suctorial disc (s. </.), the 

 central papillae of which are arranged in groups. The opercular 

 fold (Ji. op.} is very large, covering the arches behind. A well- 

 marked groove is present between the mandibular and opercular 

 arches, but so far as we can make out it is not a remnant of the 

 hyomandibular cleft. 



The pectoral fins (Plate 34, fig. \2,pc.f?} are very prominent 

 longitudinal ridges, which, owing to their being placed on the 

 surface of the yolk-sack, project in a nearly vertical direction : a 

 feature which is also found in many Teleostean embryos with 

 large yolk-sacks. 



