88 "WHITEAVES ON FOSSIL FISHES FEOM 



lengthen and increase in nnmber by bifurcation and division, so that, at the posterior 

 termination of the fin, they are abont five times as numerons as at its anterior end. 

 Each fin-ray, also, is divided through its entire length by transverse articulations into 

 rectangular joints, which are about twice as long as broad, at least in the anterior half 

 of the fin. In the largest specimen of Eusthenopteron known to the writer (No. 1), the 

 maximum length of one of the pectorals is eighty-five millimetres, and the greatest 

 breadth approximately sixty-eight. In this specimen the longest fin-rays are about one 

 millimetre and a half broad at the commencement, and at their termination f)osteriorly 

 they average from one-fourth to one-fifth of a millimetre in breadth. At and near their 

 commencement, also, they are divided into joints which average three millimetres in 

 length, but at their posterior termination the length of the joints averages from one-half 

 a millimetre to a whole one. 



The structure of the ventrals in Eusthenopteron appears to be essentially similar to 

 that of the pectorals. The only specimen in which any of the bony supports of the 

 fin-rays of the ventral fin are lîreserved is No. 4 and even in it only two of these bones, 

 which appear to correspond with the last of th* upper series (No. 3) and the last of the 

 lower (No. 6) in the pectoral, are A'isible. Of these, the upper one, which is truncated 

 at both ends, is broader and shorter than the lower one, and both are very similar in 

 shape to the corresponding bones in the pectoral. The pelvic bones are not preserved in 

 any of the specimens of Eusthenopteron that the writer Jias seen. The rays of all the 

 fins, including those of the tail, appear to have been constructed on the same plan. 



The Azygos Fins. — The external form and relative positions of these fins have 

 already been described and their component rays apparently present no special pecu- 

 liarities. The bony supports of the first dorsal are still unknown, at least td the 

 present writer, and those of the second dorsal and anal fins are best shewn in one of the 

 original types, in which a considerable portion of the posterior end of the endoskeleton, 

 with its lateral appendages, is beautifully preserved and upon which the genus was 

 largely based. In this specimen, it should be premised, the bony supports of the second 

 dorsal, as a whole, are slightly displaced, but their relative positions are unaltered. The 

 three osselets from which the fin-rays of the second dorsal simng, although slightly un- 

 equal in breadth at their flattened and rather broadly expanded outer extremities, are 

 nearly equal in length. The still more broadly expanded and laterally compressed 

 apophysis to the upper and posterior margin of which these osselets are articiilated, 

 forms in its tixrn the outer half of a much larger and probably interspinous bone which 

 narrows very abruptly a little below its midlength, especially anteriorly, and is obliquely 

 truncated at its inner termination, which is very slightly expanded. It seems highly 

 probable that this bone was originally articulated to one of the modified neurapophyses 

 of this part of the vertebral column, for the neural spine which immediately precedes it 

 (unlike any of those by which it is itself preceded, which are acutely pointed above) is 

 obliquely truncated at its slightly expanded upper and outer extremity, as if to form an 

 articulating surface of attachment. The three osselets from which the fin-rays of the 

 anal spring are similar in shape to the corresponding bones in the second dorsal, but the 

 anterior seems to have been a little the shortest of the three. The apophysis to the outer 

 margin of which these osselets are articulated, is also very similar in shax)e to that of the 



