20 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



most constricted portion of the centrum. In Lepisosteus it extends- 

 in each direction to the end of the centrum. In Amia the fibres- 

 described in the preceding paragraph appear to pass right through 

 the modified band of the sheath. 



3. DISCUSSION OF THE VERTEBRAL COLUMN OF FOSSIL FISHES. 



Having now considered the structure of the vertebral column of 

 the living Amia, it is proper to determine what light we may obtain 

 regarding its peculiarities by a consideration of such fossil forms as 

 may be related to it. For a presentation of the facts relating to the 

 Amioid fishes the reader is referred to Dr. Zittel's " Handbuch der 

 Palseontologie, " vol. iii. Through Amia, Eurycornms, Callopterus, 

 Caturus, and related genera, we are apparently led back to more and 

 more primitive arrangements. From a study of these forms attempts 

 have been made to reach an explanation of the mode of development 

 of the vertebrae of these fishes. That adopted by Dr. Zittel may be 

 thus stated. The hypocentrum (originally of two lateral halves) is 

 developed on the lower side of the notochord in each myomere, and 

 supports the lower arch. On the upper side of the notochord are devel- 

 oped two pleurocentral plates, which may also become consolidated 

 into one piece. Such an arrangement may be found in Caturus. In 

 a more advanced stage of development the hypocentrum grows fur- 

 ther upward on the sides of the notochord, while the extremities of 

 the pleurocentrum grow downward. The borders of the two ele- 

 ments may at length come into contact, and thus cover in the whole 

 surface of the notochord. Such a condition may be seen in some por- 

 tions of the vertebral column of Caturus and in that of Callopterus. 

 Further growth of the plates results finally in the upper ends of the 

 hypocentrum meeting above the notochord, and thus forming a com- 

 plete ring. In a similar way, the pleurocentrum of each myomere 

 forms another ring. We find such rings in their complete develop- 

 ment in the middle region of the tail of Amia. 



On the other hand, Ludwig Schmidt appears to regard the two 

 rings in each myomere as the primitive condition, and each of these 

 as a distinct vertebral centrum, the one with, the other without, 

 neural arches. As to the manner of development of the vertebral 

 centra of the dorsal region, Schmidt presents two contradictor} 7 views. 

 He at first states that these vertebrae must be regarded as resulting 

 from the coalescence of two such vertebras as are found in each myo- 

 mere of the tail (56, pp. 755 and 760). And he presents as evi- 

 dences and illustration of this fusion the varying number of com- 

 plete vertebrae in the anterior tail region and the occasional fusion. 



