VERTEBRATA. 7 
J. Skull imperfectly developed and without jaws; paired fins un- 
developed, with no shoulder girdle or pelvic elements ; a single 
median nostril ; am Petes skin naked ; skeleton car- 
tilaginous. . . . * 7 . « Cyciostomy, D. 
Jf. Skull well developed, nd vie — stioaldes girdle and pelvic 
elements developed; nostrils not median. 
g. Limbs developed as rayed fins (rarely abortive) ; rayed fins nor- 
mally present on’ the median line of the body; respiration 
throughout life by means of gills; lungs usually not developed. 
Piscrs, I. 
gg. Limbs not developed as rayed fins, but, if present, having the same 
skeletal elements as in the higher vertebrates; respiration in the 
adult chiefly accomplished by means of lungs, the gills usually 
not persistent; skin usually naked. . . . . Barracuia, F. 
ee. Respiration performed throughout life by means of lungs, the gill 
slits disappearing before birth. 
h. Mammary glands not present; diaphragm incomplete; a single 
occipital condyle; oviparous (or sometimes ovoviviparous), 
the young hatched from a rather large egg. (SAUROPSIDA.) 
i, Exoskeleton developed as scales or bony plates; blood cold; 
heart with three (rarely four) cavities. . . Repriurs, G. 
ti. Exoskeleton developed as tpi: blood warm; heart with 
four cavities... . we & . . . Aves, H. 
hh. Mammary glands rseenty the yous developed within the body 
from a minute egg (except in the Monotremata), and nourished 
for a time after birth by milk secreted in the mammary glands; 
exoskeleton developed as hair; two occipital condyles; dia- 
phragm complete; heart with four cavities; blood warm. 
Mamma.u, I. 
Of these classes, the Tunicata (A) and the Enteropneusta (B) are 
excluded from the plan of the present work. The Tunicata are all 
marine forms, of small size, the larger species being familiarly 
known as “Sea Squirts,” “Sea Peaches,” and “Sea Pears”; but 
the most of them are without common names. A considerable 
number of species, representing several families, are found on our 
Atlantic coast. The Enteropneusta consist of the single genus 
Balanoglossus, a worm-like creature, of which two or three species 
are found on our coasts. They reach a length of six to twelve 
inches. They have been considered as worms having possible 
affinities with the Echinoderms, but the recent studies of Mr. 
William Bateson seem to show conclusively that their place is 
among the Chordata. 
Leaving these groups aside, we take up 
