344 THE AMERICAN NATURALIST. [VoL. XXXII. 
Those ossifications starting from the bases of the arches, upper 
and lower of each segment, extend themselves (Fig. 2) and 
finally unite and form a ring of bone around the notochord, and 
lying outside of its sheath. This ring is the so-called “ hypo- 
centrum,” supposed hitherto to arise from the upward growth 
of elements situated on the lower side of the notochord. The 
ossifications connected with the four intercalated cartilages 
(Fig. 3) spread until they meet, and thus produce another ring 
of bone around the notochord, the so-called “ pleurocentrum,”’ 
hitherto held to be the product wholly of elements situated on 
the upper side of the notochord. It thus becomes evident that 
each of the two rings of the muscular segment has another 
pair of elements entering into its composition besides the pair 
usually attributed to it. It may further be seen that the 
element which develops from the bases of the upper arch, the 
epicentrum, may easily be mistaken for the true pleurocentrum, 
which in reality has its origin in the upper intercalated 
cartilages. 
In the abdominal region of Amia I found that the lower 
intercalated cartilage is apparently wanting. On the other 
hand, the upper ones, which give rise to the 
pleurocentra, become greatly expanded, push 
themselves under the bases of the upper arches 
immediately behind, and suppress the ossifica- 
tions that might be expected to arise there 
(Fig. 4). The pleurocentra thus come to sup- 
port the upper arches, and, growing downward, 
coalesce with the up-growing hypocentra to form 
the completed vertebral centrum. It will be 
seen that in Amia there are three kinds of ver- 
‘ie, pleurocentrum tebral rings, víz., those formed from the union 
bearing the neural of the bases of the upper arches with the bases 
of the lower arches, those from the intercalated 
cartilages, and those partly from bases of lower arches and 
partly from the upper intercalated cartilages. 
Now, how does ossification take place in the vertebral column 
of Lepisosteus, the living representative of the Aetheospondyli? 
Certain stages have not yet, so far as I am aware, been worked 
