22 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



Dapedius is regarded as having possessed composite vertebrae. These 

 meager accounts of the vertebral structures of these fishes make it 

 probable that all the species of the families, so far as they possessed 

 anything representing vertebral centers, had the two elements, pleu- 

 rocentra and hypocentra. And the same having been shown to be 

 true, or highly probable, of every family of the order Lepisostei, 

 we are, I think, justified in reaching the conclusion that whenever 

 ossification of the vertebral column has begun in members of this 

 group, it has been through the formation in each somite of a lower 

 piece or pieces, on which rest the lateral halves of the haemal arch ; 

 while on the upper side of the notochord upper pieces of bone have 

 been deposited in the region of the bases of the upper arch ; that is, 

 pleurocentra and hypocentra are normally constituents of the ver- 

 tebral centrum. 



Of the condition of the ancient Crossopterygia, as regards the 

 vertebral column, we have little knowledge. Prof. Cope (15, p. 19} 

 states that in Ectosteorhachis (Megalichthys) the vertebrae are repre- 

 sented by annular ossifications resembling those of Cricotus. There 

 appears, however, to be only a single ring in each myomere. 



From the foregoing survey of the condition of the vertebral 

 column in the different families of the so-called Ganoids, one may 

 easily become convinced that originally the ossified vertebral centra 

 of all the species that possessed such were composite in their struc- 

 ture; that is, each centrum included in its composition a pleurocen- 

 trum and a hypocentrum. Each of these two elements was doubtless 

 itself, at an earlier period of its history, double. If the centrum is 

 in any given case simple in structure, this has probably resulted from 

 the coalescence at some time in the animal's life of the two elements, 

 or possibly sometimes from the suppression of one of them. If we 

 find in one fish the notochord surrounded in one portion of the body 

 by a solid vertebral body, and in another portion by two complete, 

 but distinct, rings; or in another fish the notochord in one region 

 protected by pleurocentra and hypocentra, while in another region 

 there are mere rings, it appears quite improbable that these different 

 structures have no genetic connection. It is equally improbable that 

 in species of the same family, or of closely related families, the ver- 

 tebral centers originated sometimes as mere tubular incrustations of 

 the notochord, sometimes as double rings, and sometimes as pleuro- 

 and hypocentra. Some one of these structures must have been 

 the fundamental one ; and if so, then the elements which have been 

 called pleuro-and hypocentra must be regarded as the most primitive. 



To what extent do the conclusions we have reached apply to the 

 fishes which we call the Teleostei ? 



