74 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
apex terminates beneath the cardinal groove (cd.g.) by becoming suturally united with 
the oblique posterior or inner margin of the dorsal lamina (fig. 3). The anterior margin 
of each ossification coincides with, and indeed, appears to form, the oblique lateral 
ridge of the complex centrum. Traced posteriorly, each superficial ossification gradu- 
ally contracts and partially exposes the lateral surface of the fifth vertebral centrum, 
but is, nevertheless, continued as a narrow splint of bone along the ventro-lateral 
edge of the succeeding centra as far as the seventh (fig. 3). The free ventral margins 
of the two ossifications and their splint-like prolongations project downwards beyond 
the ventral surfaces of the different vertebral centra with which they are in relation, 
so as to form the lateral boundaries of a deep but narrow median groove extending 
from the first to the seventh vertebra (figs. 3 and 23, a.g.). The lateral margins of 
the anterior extremity of the groove are formed by the paired accessory articular 
processes of the basioccipital and the centra of the first and complex vertebrae. The 
groove has a fibrous ventral wall, in part, at least, formed by the median portion of 
the dorsal wall of the anterior chamber of the air-bladder, and in the canal, so 
constituted the anterior section of the dorsal aorta is lodged. In the dry skeleton 
paired nutrient foramina may be seen In the dorsal wall of the groove, leading into 
the different vertebral centra ; of these foramina one pair only belongs to the complex 
centrum (fig. 3). Between the dorsal edge of each superficial ossification and the root 
of the transverse ])rocess of the fourth vertebra, the lateral surface of the complex 
centrum is traversed by a deep groove which, anteriorly, is converted for a short 
distance into a complete canal by the extension of the dorsal lamina from the radial 
nodule to the ventral surface of the transverse process (fig. 3, cd.g.). Through this 
groove and short canal, the posterior cardinal vein passes forwards to its junction with 
the anterior cardinal in front, the union of the two v^eins to form the Cuvierian duct 
taking place in a somewhat triangular fossa situated immediately anterior to the 
radial nodule. The attenuated anterior lobes of the mesonephros accompany the 
posterior cardinal veins as they extend forwards to their terminal lymphoid enlarge- 
ments in front of the air bladder. In correspondence with the asymmetrical 
development of these veins the left cardinal groove is much narrower than the right. 
It will be noticed (fig. 3) that the dorsal edge of the posterior half of each super- 
ficial ossification forms the ventral lip of the cardinal groove of its side, and also, in 
conjunction with the hinder margin of the dorsal lamina, gives rise to a A-shaped 
ridge, the two arms of which converge from the ventro-lateral margins and sides of 
the fifth and sixth vertebral centra, and from the ventral surface of the transverse 
process of the fourth vertebra, eventually uniting on the root of the latter, near the 
junction of the body of the fifth vertebra with the complex centrum. These con- 
verging ridges coincide with the line of attachment of the dorsal margins of the 
primary transverse septum of the air-bladder to the skeleton. 
It is mainly owing to the growth of the investing superficial ossification and their 
downward projection beyond the ventral margins of the anterior vertebral centra that 
