70 
PROFESSORS T. W, BRIDGE AND A. C. HADDON 
become indistinguishably fused, with the final result that in the adult the three 
centra are anchylosed into one apparently simple centrum. Hence it follows that in 
Amiurus the first intervertebral enlargement of the notochord is between the basi- 
occipital and the body of the first vertebra, the second between the latter and the 
second vertebral centrum, and the third enlargement between the centra of the fourth 
and fifth vertebrae. The same observer has also pointed out that the neural canal 
over this composite centrum is bounded by a continuous tube of membrane bone, 
which originates near, and apparently replaces in the adult, the rudimentary cartila- 
ginous neural arches of the third and fourth vertebrae in the embryo. The neural 
spines of the two vertebrae, on the contrary, are free at their distal extremities, 
although proximally they are continuous with each other and with the tube of the 
membrane bone. 
For convenience in descriptive anatomy we venture to propose for the confluent 
second, third, and fourth vertebrae the term “complex vertebra.” The extent to 
which the second vertebral centrum enters into the composition of the complex 
centrum is not altogether clear. According to Ramsay Wright (42), it “serves 
apparently merely to deepen the anterior concavity of the third vertebra, although in 
horizontal sections through Fish of 3 to 4 cm. in length the second and third 
vertebrae appear of almost equal size.” But, from the description and figures, it would 
seem that the second and third centra together form the anterior third of the 
complex centrum, the remaining two-thirds representing the body of the fourth 
vertebra. On similar grounds we may perhaps regard the anterior third of the 
continuous neural arch of the complex vertebra, which is perforated for the exit of 
the dorsal and ventral roots of the fourth pair of spinal nerves, as belonging to the 
third vertebra, and the posterior two-thirds, similarly perforated for the transmission 
of the fifth pair of nerves, as representing the neural arch of the fourth vertebra. 
Returning to Macrones nemurus, we find that that the centrum of the complex 
vertebra is much larger than any of the normal centra, and has its posterior concavity 
much deeper than the anterior (figs. 2, 4, and 5, c.c.). On the ventral surface of the 
centrum, near its anterior extremity, there is a pair of nutrient foramina (figs. 2 and 
5, n.f.). In some Siluridm, but not in Macrones, there is a second pair behind the 
first, and in that case it is evident that the two pairs are the normal nutrient 
foramina for the third and fourth vertebrm. The anterior extremity of the complex 
centrum is provided on its ventral surface with a pair of stout accessory articular 
processes, which incline forwards towards the corresponding processes of the 
basioccipital, but remain separated from them by the more slender articular processes 
of the centrum of the first vertebra (fig. 5). The centrum of the fifth vertebra is of 
still greater length, being about equal in this respect to the complex and first centra 
taken together (fig. 2, v}). Its anterior and posterior concavities are approximately 
equal in size, although the latter may sometimes be slightly the deeper of the two. 
Both the complex and fifth centra are not only greatly elongated but also laterally 
