42 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 







ossified rings have ever existed. The pleurocentrum and hypocen- 

 trum push themselves over the notochord until they come into con- 

 tact and form triangles on the sides of the notochord. Whether such 

 elements ever continue to grow around the notochord and produce 

 complete rings I do not know. In view of what happens in Amia, it 

 would appear to be most reasonable to deny this until the evidence is 

 made unquestionable. It is interesting to note that in both Caturus and 

 Callopterus, according to Zittel's account, no rings appear in any por- 

 tion of the body. Euthynotus possibly furnishes us with a case in 

 which the hypocentrum has grown around a considerable portion of 

 the notochord, while the pleurocentrum is reduced to small propor- 

 tions, but the epicentrum is probably present and the genus therefore 

 belongs nearer Eurycormus. With the same elements to start with, 

 the various genera have developed a considerable variety of struc- 

 ture in their vertebral structure, and it will require much investigation 

 to discover all the relationships. 



Attention ought to be called to the fact that the pleurocentrum 

 and the ring containing it belong to the anterior end of the vertebra, 

 or the vertebral complex, into which it enters. In the middle 

 tail region of Amia, whenever, with rare exceptions, which may be 

 regarded as abnormal, the two vertebral rings unite, the one contain- 

 ing the pleurocentrum is anterior. In the dorsal region, it is the 

 intercalated cartilages placed in front of each arch which push them- 

 selves backward beneath the latter and unites with the hypocentrum 

 to form the centrum. It is necessary to keep this in mind in exam- 

 ining the text and figures of Dr. Zittel's Handbuch, in which the 

 statement is sometimes made that the pleurocentrum is the hinder 

 element. 



I have already, on page no, called attention to Schmidt's expla- 

 nation of the development of the dorsal vertebrae of Amia. My invest- 

 igations on the young of Amia have demonstrated that neither of his 

 hypotheses is true. There is no fusion of two such simple rings for 

 each myomere as are found in the tail. Furthermore, the pleurocen- 

 trum is not suppressed by the growth of the hypocentrum. Such a 

 process as he describes would make of the centrum simply a hypo- 

 centrum. On the other hand, in Amia, the pleurocentral element 

 forms the whole upper half of each dorsal vertebra. 



7. FURTHER DISCUSSION OF THE VERTEBRAL STRUCTURE OF THE 



TELEOSTEI. 



We may return for a moment to the consideration of the verte- 

 bras of our living Teleosts. Does the pleurocentrum enter into the 



