540 H. H. SWINNERTON. 



distinguish Gasterosteus, Syngiiathns, and Siplionostoma, 

 from the sahnon, trout, and pike, seem to be associated 

 with the production of a small mouth ; and the differences 

 between the two types may be summed up by saying that in 

 the latter the symplectic and metapterygoid processes remain 

 short whilst the gape enlarges, and in the former those parts 

 elongate whilst the gape remains small. 



Unlike the cranium, the visceral skeleton is not wholly 

 devoid of bones at this stage, for the dentary, maxilla, and 

 operculum, are already present. Of these the last two 

 remain thi'oughout life as purely dermal elements ; but the 

 first, though now merely a delicate sheet of bone lying out- 

 side of Meckel's cartilage, becomes later on closely related 

 to this. 



Stage III. — In the branchial apparatus (PI. 28, fig. 15) 

 each of the original rod-like arches has now begun to break up 

 into segments. In the first three arches the flattened ventral 

 extremities form the hypobranchials; in the fourth it never 

 becomes separate. The epibranchial of the first ends freely; 

 that of the second articulates partly with the corresponding 

 phaiyngo-branchial, and partly with the side of the large 

 element behind. This latter also gives attachment to the 

 third and fourth epibranchials, and may be regarded as the 

 representative of the corresponding pharyngo-branchial. The 

 connection of the second epibranchial to this is probably 

 secondary, and indicative of a tendency to complete reduction 

 of the second pharyngo-branchial with transference of its 

 function to the one behind. 



Both representatives of the pharyngo-branchial series are 

 now armed with teeth, and the hinder one has already 

 become partially ossified. 



The copulare commune (fig. 15, hr. h. l—^) has extended 

 further backwards, and is now in contact with the basi- 

 branchial of the fourth arch {br. h. ^), which has undergone 

 no change. 



In the hyoid arch, the basihyal, and hypohyal, are prac- 

 tically the same as before ; but the combined ceratohyal and 



