412 The American Naturalist. [May, 
rected outwards, were developed to a varying extent along the 
lines of these folds. 
3. The longitudinal folds were interrupted variously, leaving 
two prominences on each of the lateral folds. 
4. The anterior prominences (or limbs) increased more than 
the posterior. 
5. The bases of the cartilaginous supports coalesced according 
to the needs of the various isolated parts or fins. 
6. Occasionally the dorsal radials coalesced, and sought cen- 
tripetal adhesion to the axis. 
7. More constantly the hinder laterals coalesced, and finally 
prolonged themselves inwards by mesiad growths till such a pelvic 
structure as is found in Squatina was developed. 
8. The pectoral rays also coalesced proximally, and, in search 
of a support, shot dorsad and ventrad to avoid the visceral cavity- 
They thus attained the axis above and came together below. 
9. The lateral fins, as they were used for support, became 
elongated and segmented, till probably the propterygium, but 
possibly the mesopterygium, became the axis of the digit-bearing 
limb. 
10. Pre-existing cartilages were enlarged, or new ones were 
developed, at the distal ends of the paired limbs, till the cheirop- 
terygium (leg) was developed. 
11. The pelvic limb acquired a solid connection with the spine 
from the need of a firm support on land. 
12. The pelvic limb became elongated. 
These changes all came about from external causes acting oP 
the plastic animal organism, and the limitation of the paired 
limbs to four was determined solely by locomotive convenience. 
Prof. Mivart gives Mr. Thacher the credit of the first pro- 
mulgation of the ideas he has adopted, but does not follow the 
latter in his conclusion that median fin-rays are never derived 
from neural spines, nor neural spines frog. fin-rays, as he con- 
siders this point to be unsettled, and only capable of proof by 
the study of their development. 
The discovery of the structure of the pectoral and ventral fins 
in the Ichthyotomi by Sauvage and Döderlein, has substantiated 
