OCT. 1695. VERTEBRAL COLUMN OF AMIA HAY 7 



anterior is but little more than half the length of the twenty-third. 

 Schmidt's description, but not his figure, would lead one to suppose 

 that the first two vertebrae behind the head are shorter, when com- 

 pared with the succeeding ones, than they really are. 



In transverse section the dorsal vertebrae are somewhat broader 

 than high, the perpendicular axis of the first dorsal being about 

 eighty-five per cent of the transverse axis. As we move backward, 

 the form changes somewhat, so that sections become more and more 

 nearly circular, the hindermost dorsal body being nearly perfectly so. 

 The caudal centra, on the other hand, become more and more com- 

 pressed towards the tip of the tail. Thoroughly macerated verte- 

 brae, as well as sections through decalcified vertebrae in various 

 regions, show that the notochord has not suffered complete constric- 

 tion, but runs continuously through the vertebral column. 



It is no unusual thing to find the pleurocentrum and hypocen- 

 trum of some of the myomeres of the middle portion of the tail con- 

 solidated. Stannius (58, p. 21) had observed this union of elements, 

 and that it occurred at different points in different individuals. 

 Schmidt has called attention to the same phenomenon, and has 

 figured such a resulting vertebra. In such cases there is the closest 

 possible resemblance between the vertebra so resulting and one of 

 the anterior caudal vertebrae. At the same time there can be no 

 doubt concerning the complete homology of an anterior caudal verte- 

 bra and any of those of the dorsal region. 



On the other hand there may occur a union of the elements of 

 different vertebrae. A specimen in my possession shows an evident 

 case of the consolidation of the hypocentrum with both the pleuro- 

 centrum in front of it and the one behind it. 



Even if we should not be able to find in the middle tail region a 

 vertebra formed by such apparently abnormal union of pleurocen- 

 trum and hypocentrum, we might find it instructive to compare these 

 elements of any myomere with a posterior dorsal and an anterior 

 caudal vertebra. The general form of the two elements taken to- 

 gether is the same as that of the simple vertebral body. In the case 

 of the middle tail segments, the upper and lower arches rest 

 on the hinder element, the intercentrum. In the dorsal region the 

 arches repose on the hinder half of the vertebral body. In 

 "both the middle and the anterior tail-regions the upper and the lower 

 arches are separated by suture from the corresponding bodies. If 

 we should form our conclusions regarding the composition of the dor. 

 sal and the anterior vertebrae from what a macroscopic view of 

 the parts affords, we would, I think, conclude that they have been 



