li^lO.] CAUDAL FIN- OF THE TEL::o.S TOMI. 601 



1. Leptocephalushrevirostrl^. (Plate XLVIJ J. liir. Tl.) 



It is perhaps somewhat unfortunate that in the l^cptocpphaJitK 

 all essential skeletal structures are already j'vesent ; in fact, there 

 is very little difference between it and the elver four inches in 

 length. Tliei-e is very cleai-ly shown a comparatively lono- uro- 

 style slightly directed upwai'ds ; this urostyle is not fused with 

 the hypural bone innnediately ventral to it. In my specimen 

 this hypural, the distal end of which extends beyond the urostyle, 

 was already a single cartilage, though in oldei- specimens, a verv 

 distinct cleft was frequently to be seen. Three of the four fiii- 

 rays attached to this hypural were bitid distally, though I believe 

 the number of distally bifurcated rays is a matter of accident. 

 The penultimate vertebra carries the characteristic U-shaped 

 hypural of the Apodes, I'epresenting two pieces fused proximally. 

 The antepenultimate vertebra is a double structure, and cai-ries 

 two hfemal 2)rocesses, the posterior of wdiich bears a fin-ray and 

 shows a definite trace of being formed by the union of a radial 

 and a hjemal ai'ch ; this has already been drawn attention to by 

 Kyder. The double vertebra mentioned is not, as Ryder seems 

 to suggest, a constant feature, but certainly is very common. 

 Dorsally, no epural bones ai'e present ; eveiy ray is su])ported by 

 a radial. In the only figure of the caudal fin of a young eel I a.i'u 

 aware of, namely that of Ryder's, the neural arches are not truly 

 represented ; in reality they are rectangular plates, equal in 

 length to the vei'tebi'je to which they are attached ; even the last 

 retains its rectangular shape, covering up ui'ostyle and overlapping 

 the last hypural to some extent. The spinal cord can be traced 

 even among the fin-rays beyond the distal end of the last hypui'al ; 

 this fact is evidence of considerable abbreviation. 



2. The Elver. (4^ ins. or about 12 cm, long.) (Plate XLVIII. 

 fig. 10.) 



This stage is next taken as being the last in which the ui'ost^de 

 can be traced ; the last hypural has already encroached upon it 

 dorsally, while in specimens five inches long it has vanished, and 

 the last hypural and the last vertebral segment have become a 

 single piece. ISTeural spines have appeared on all the neural aiches 

 except the last two, and a thickening indicative of a spine is seen 

 there also. The spinal cord does not extend among the fin-rays as 

 in the Leptoceplialns. All other features are much the same 

 as in the Leptocephaliis stage, though naturally more pronounced. 



3. Common Eel — adalt. (Plate XLVIII, fig, 11.) 



After the preceding stage, and sometimes earlier, the last two 

 neural arches often fuse into one. But the main featui'e fo be 

 noted in the adult is the complete aljsoi'ption of the last vertebi-al 

 segment, including centrum and urostyle, by the last hypural ; all 

 eftbrts to I'eveal the urostyle by clearing agents have failed ; the 

 union is complete, hence, what appears in the adult to be hypural 



