346 THE AMERICAN NATURALIST. [Vouw. XXXII. 
of our Lepisosteus. Their vertebral centra have probably been 
produced by the fusion of the ossifications arising in the upper 
arches with those of the lower arches. We do not need to 
disturb these genera. 
Among the Protospondyli we find the nearest allies of the 
Amiidæ in the family Eugnathida. Here we discover abundant 
evidences of the presence of true pleurocentra. They are 
shown to be such by the varying positions of the neural arches 
on them. Neural arch and epicentrum, being ossified portions 
of the same arcuale, to use Gadow’s convenient term, remain in 
direct contact, even when not codssified. When the epicentrum 
Fic. 6. Fic. 7. Fic. 8. 
Fic. 6. — From tail of Eurycormus. After Zittel. Shows the arch-bearing rings formed by union 
of epicentra, ef.c., and hypocentra, Ayf.c.; also the archless rings formed by union of 
hm. 
Fic. 7. — From abdominal region of Eurycormus. After Zittel. Hæmacentra wantin 
Fic. 8.— From abdominal region of Callopterus. After Zittel. Both hamacentra sail epicentra 
are aborted, leaving only the pea pi.c., and the hypocentra, Ayf.c. This represents 
ne stage in the abdominal region of Am: 
is not developed, the pleurocentrum may wholly or only partially 
push itself under the arch. Whenever we find two distinct 
rings for each segment, the upper and the lower arches are 
connected with the same ring, as shown in Fig. 6, which repre- 
sents the vertebræ in the tail of Eurycormus. In the abdominal 
region of Eurycormus the arch-bearing ring is apparently com- 
plete and carries the arches, while the pleurocentra are wedged 
in between them (Fig. 7); but the hæmacentra, which make up 
the lower half of the archless rings in Fig. 6 (Am.), are wanting. 
A little further reduction of the epicentra leads to the condi- 
tion found in Callopterus (Fig. 8) and some of the species of 
Caturus. We may be sure that species of Caturus which have 
not yet revealed ossified vertebral elements nevertheless pos- 
sessed these elements in some form, though possibly only 
