422 The American Naturalist. [May, 
of embryo ray in the lophocercal stage, natural size, from a specimen 
taken near Wood's Holl, Mass. Fig. 3, Lophocercal tail of young flounder 
6mm. long. Fig. 4, Lophocercal tail of young flounder a little older than 
the preceding, beginning to show a slight upbending of the notochord, and 
the first trace of the permanent caudal lobe (fc) and opisthural lobe (2%). 
Fig. 5, Caudal lobe of a somewhat older flounder, showing indentation and 
definition of permanent fin-rays. Fig. 6, Specimen with tip of notochord 
still more reflexed than in the preceding; permanent caudal and opisthural 
lobes somewhat more distinct. Fig. 7, Permanent (c) and opisthural lobes 
(of) now form a sharp angle where they join ; distinction between perma- 
nent and embryonic rays well marked. Fig. 8, Permanent caudal as long 
as opisthural lobe (of). Fig. 9, Cartilaginous Sippotis of fin-rays are now 
strongly developed; the end of the chorda has begun to degenerate and 
approximate the position which it will occupy permanently as the urostyle. 
Fig. 10, The caudal has become more rounded, the opisthure (2%) is almost 
wholly absorbed, and the notochord has suffered atrophy somewhat, and 
now presents a still.closer approximation to the form of the urostyle of the 
adult. Figs. 3-10 inclusive after A. Agassiz. 
PLaTE XVI.—Fig.1, Caudal skeleton of Coccosteus after Pander; z% and 
hy, epural and hypural elements, all of which do not bear rays, but as in 
Pterichthys, extended out only so far as the scaly covering of the tail; æ, 
dorsal ; a, anal fins. Fig. 2, Polypterus bichir, caudal skeleton, from Agassiz’s 
Poissons Fossiles, modified from Kölliker; ef, styliform ray-bearing and 
nodular non-ray-bearing interspinous epural elements; ¢/, neural spines; 
Ay, hypural ray-bearing elements. Fig. 3, Lepidosteus, caudal skeleton 
than in the preceding. Fig. 4, Platysomus restored, after Agassiz’s Poissons 
Fossiles; ef and Ay epural ind hypural pieces ; x, urochordal end of skele- 
tal axis, which was mainly notochordal. Fig. 5. Lepidosteus, tail of young 
specimen 11 cm. long, from Balfour and Parker; cd, permanent caudal; 
f, eradiate fin fold of opisthure; of, opisthure; my my, its myotomes. 
Fig. 6, Lepidosteus, young, 21 mm. long, side view; dissected and magni- 
fied so as to show its structure at this stage; ef and Jy, epural and hypural 
cartilaginous rudiments of the neural and hæmal arches; ch, chorda; cå, its 
opisthural portion, which afterwards becomes partially aborted and in- 
cluded in the upper part of the tail ; cd, tip of fold, which becomes the per- 
manent caudal; of, opisthural lobe of the larval tail; #7, lophocercal fin- 
fold, which contains horn-fibres throughout its extent, 4f. After Balfour 
and Parker. Fig. 7, Magnified view of the caudal skeleton of a young 
Cyprinodont, Gambusia, 4% of an inch long, and which was removed from 
the ovarian follicle in which it developed ; ch, chorda dorsalis; 2% and Åy, 
epural and hypural cartilages; ms, medulla spinalis; 777, rays. Fig. 8, 
Centrina salviani, caudal skeleton of adult; ef and hy, as before ; va, ver- 
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