540 PROF. T. W. BBIDaE OlST THE MESIAL 



bones " whicli lie behind the radial elements of the dorsal fin 

 and continue the series as far as the caudal fin, and had pre- 

 viously been overlooked by Franque. In one of two specimens I 

 examined four such structures were present and in the other 

 three, in the form of elongated but extremely slender ossicles. 

 As to the nature of these structures, there can be no doubt that 

 they are the persistent proximal segments of a series of vestigial 

 radial elements, and indicate the primitive continuity of the 

 dorsal and caudal fins. The discrepancy in numbers in the 

 different specimens examined is probably due to the well-known 

 variability of such vestigial structures, of which yet another 

 instance may be mentioned. In Shufeldt's figure of these vestiges 

 {I. c, pi. ix. fig. 25) they are represented as without dermal rays, 

 but, curiously enough, in one of my specimens the last two of 

 the series were related distally to two small broadly V-shaped 

 vestigial fin-rays, which were wholly imbedded iu the subcu- 

 taneous connective tissue ; in the second specimen no trace of 

 these structures could be found. 



LEPIDOSTEIDiE. 



Lepidosteus osseus. 



Dorsal fin. — In this Ganoid the dorsal fin is situated imme- 

 diately anterior to the anal fin, and consists of eight radial 

 elements (PI. XXI. fig. 3), supporting eleven soft dermal fin-ra\ s. 

 The 2nd to the 8th inclusive are trisegmental, and in shape and 

 in their relations to one another and to those of contiguous 

 elements the different segments closely resemble tliose of Amia. 

 The mesial segments, like the proximal, are all well ossified, with 

 the exception of that belonging to the second radial element, 

 which, as is. also the case with all the distal segments, is carti- 

 laginous. Th.e first radial element (r.eJ-) has a much larger 

 proximal segment than any of the others, and the simple elon- 

 gated nodule of cartilage which is attached to its distal extremity 

 apparently represents a distal segment. Of the eleven fin- rays, 

 the first three are supported by the distal segments of the first 

 radial element, and the tenth and eleventh by the corresponding 

 segment of the last element. The remaining fin-rays are each 

 supported by a distal segment, precisely as iu Amia. 



The fact that the proximal segment of the first radial ele- 

 ment is larger than any of the other proximal segments, and 

 is related to three dermal rays, suggests the possibility of the 

 fusion of certain of the anterior supporting elements of the fin. 



