E. T. NEWTON ON FISHES' TAILS. 95 



tail is concerned, that the Teleostei may have so descended, for it is 

 quite conceivable that a form of tail like that of Lepidosteus might, 

 with comparatively little change, become such a homocercal form as 

 we get in the sprat. If the Teleostei have been thus evolved, the 

 links of the chain connecting the two groups are still unknown. 



Before these problems can be solved there is much work to be 

 done both in working out the structure of fossil forms and also 

 in the investigation of the embryology of recent Teleostean and 

 other fishes. 



DESCRIPTION OF PLATE III. 



Fig. 1. — Tail of a young Sprat (Harenga sprattus), with the four preced- 

 ing vertebrae ; enlarged about 25 diameters. The fin rays are 

 omitted for the sake of clearness, na, neural arches; na*, 

 modified neural arch ; ha, haemal arches ; 1 to 7, series of 7 

 hypural bones, sp. 1, one of a pair of splint-like processes 

 arising from the first upwardly directed vertebra, and protecting 

 part of the unossified notochord ; sp. 2, sp. 3, two other splint- 

 like bones, which, with corresponding bones on the opposite 

 side, still further protect the notochord ; nch, the unossified 

 notochord, extending beyond the hypural bones ; ep, epiurals; these 

 appear to be serially homologous, with the spines of the neural 

 arches, but are incomplete below. 



Fig. 2. — Tail of a Stickleback (G aster osteus), enlarged, after Huxley. 

 Only a portion of the fin-rays are represented. 1, 2, hypural 

 bones ; nch, notochord ossified and fixed to the border of the 

 uppermost hypural bone. 



Fig. 3. — Central portion of the tail of a Codfish (Gadus morrhua), 

 natural size, na, two hindermost neural arches ; ha, two hinder- 

 most haemal arches ; 1, the single hypural bone ; nch, the ossified 

 notochord firmly attached to the upper border of the hypural bone. 



Fig. 4. — Tail of the Bony Pike {Lepidosteus), after Kolliker, reduced : 

 nch, notochord; in front of this are shown a series of vertebrae, 

 with their neural and haemal arches, all of which become smaller 

 as they are traced backwards. 



Fig. 5. — Tail of Sturgeon (^1 cipenser), much reduced, after Agassiz. nch, 

 notochord ; above and below this are shown the large series of 

 cartilages representing the neural and haemal arches. 



