Raymond C. Osburn 193 
PUNNETT, R. C., o0.—On the formation of the pelvic plexus, with especial ref- 
erence to the nervus collector in the genus Mustelus. Tr. Royal Soc., 
London, Series B, Vol. 192; also, Zo6l. Anz., XXIII, Leipzig, 1900. 
or1.—On the composition and variations of the Pelvic Plexus in Acan- 
thias vulgaris. Proc. Royal Soc., London, Vol. LXVIII, London, 1901. 
04.—Merism and Sex in Spinax niger. Biometrika, Vol. III, No. 4, 
1904. 
Rast, C., g1.—Zur Entwickelung der Rippen und Extremitéten der Wirbel- 
thiere. Verh. Anat. Ges. Jena, 1891. 
97.—Theorie des Mesoderms. Engelmann, Leipzig, 1897. 
o1.—Gedanken und Studien tiber den Ursprung der Extremitaten. 
Zeitschr. wiss. Zool., LXX, Leipzig, 1901. 
REGAN, C. TATE, 04.—The Phylogeny of the Teleostomi. Ann. Mag. Nat. Hist., 
Vol. XIII, Series 7, 1904. 
RuGE, E., 0o2.—Die Entwickelungsgeschichte des Skelettes der vorderen Ex- 
tremitat von Spinax niger. Morph. Jahrb., XXX, 1902. 
THACHER, J. K., 77,—Median and Paired Fins, a contribution to the history of 
vertebrate limbs. Trans. Conn. Acad. Science and Arts, Vol. III, 
1877. 
78.—Ventral Fins of Ganoids. Trans. Conn. Acad. Science and Arts, 
Vol. IV, New Haven, 1878. 
WIEDERSHEIM, R., g2.—Das Gliedmassenskelet der Wirbelthiere, etc. Jena, 
1892. 
DESCRIPTION OF PLATES. 
PLATE I. 
Fics. 1 to 6. Cross-sections through various fins of Cestracion to show 
origin of concentrated mesenchyme (later giving rise to skeleton) within the 
fin-fold in every case. The difference in appearance between the concentrated 
mesenchyme in the fin-fold and the ordinary mesenchyme in the body-wall is 
clearly indicated. All the figures are from camera drawings. 
Fic. 1. Through anterior end of pectoral fin-fold shows muscle-buds (m) 
descending into the fin. From the collection of cells below it and the partially 
free space above it, the bud seems to be pushing the mesenchyme cells out of 
itS way. 
Fic. 2. Through middle of same fin as Fig. 1. 
Fic. 38. Through inferior caudal fin-fold. The axial skeleton is not yet evi- 
dent and the mesenchyme thickening is confined to the fin-fold at this stage 
as it is in the other fins. 
Fic. 4. Through superior caudal fin-fold. 
Fic. 5. Through anterior end of first dorsal fin-fold. 
Fic. 6. Through middle of the same fin shown in Fig. 5. 
PLATE II. 
Fic. 7. Cross-sections through pectoral girdle (p) and fourth gill-arch (g). 
The dotted lines connect similar points, The girdle (p) is in its first stage, 
prior to the development of the dorsal (scapular) portion, and it is still con- 
tinuous with the procartilage (b) within the fin-fold. Note that the girdle 
