EXPLANATION OF PLATE IV. 



Fia. 1. Caudal skeleton of ^miur«8 oZ&idus, from an embryo fifteen days old ; rays not 

 represented; the vertebras V, IV, III, clearly marked, II and I subse- 

 quently become fused and continuous with the urostyle m. The cartila- 

 ginous epural and hypural pieces consist of neural spines n, coalesced with 

 interneural elements i and basilar epiphysial elements or actinophores b ; 

 0,0 and cv, aorta and caudal vein embraced by the hasmal arches h. Me- 

 dulla spinalis ms embraced by the neural arches n, but degenerate pos- 

 teriorly and exserted beyond the urostyle, and resting partly on the 

 opisthural cartilage op. x 64. From a chromic acid specimen which was 

 sectionized and the outlines of the elements superimposed with the camera- 

 lucida. 



Fig. 2. Tail of an embryo of Siphostoma, showing the rudimentary hyparal pieces, evi- 

 dently comprising several coalesced elements, as is shown by the number of 

 rays r. x 96. From a transparent specimen mounted in balsam. The up- 

 ward deflection of the end of the medulla spinalis ms is also indicated. 



Fig. 3. Tail of a very young Hippocampus antiquorum, x 183, showing a very rudi- 

 mentary caudal fold cf, and a trace of the hypural cartilage hy, which is 

 partly opisthural or post-chordal. 



Fig. 4. Tail of a very young Anguilla two and one-third inches long, in the ccela- 

 canthous stage of develoj)ment, but with the interspinous elements nearly 

 entirely cartilaginous, i, interspinous elements ; /(, hajmal arches ; 6, basi- 

 lar cartilages, all discrete except the four hypural elements, which are re- 

 duced by concrescence to two in the adult. The rays from r' to r' are dichot- • 

 omons, and belong to the caudal proper, while the rays r are dorsal and 

 anal. Some epural interspinous pieces are aborted, and the last vertebra u 

 is pointed posteriorly, and represents the urostyle. The last four hypural 

 processes bear two caudal rays each, thus indicating that they each repre- 

 sent two coalesced basil.ar and interspinous elements, since there are two 

 successive systems of interspinous and basilar el6ments to each vertebra 

 anterior to the two true caudal segments. The urostyle has therefore de- 

 generated and shortened so that proximal concrescence of the median ap- 

 pendicular skeleton bearing the eight caudal rays has occurred. The last 

 three vertebrte c c u are ventrally two-spined, the last two four-spined, if 

 their hypural elements are considered to be double. Tlie last two caudal 

 centra are spineless dorsally. x 32. 

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