OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 15 



bones have not been previously observed. Shufeldt remarks (57, ex- 

 planation of plate ix.) that these bones, taken in connection with the 

 free spines (interspinous bones) found over the vertebrae immediately 

 behind the head, suggest that in the early ancestors of Amia the fin 

 was continuous from the base of the cranium to the tail. In Oligo- 

 plciirus, of the Upper Jura, which has a far shorter dorsal fin, there 

 is a series of such bones occupying the whole space between the head 

 and the dorsal fin. (Zittel, 60; iii. 231.) 



Of these epural interspinous bones I have found, in two speci- 

 mens examined, also four. The third of these bones is in close rela- 

 tion with the base of the next to the last epural fin-ray, the second of 

 those found by me. The fourth bone is closely bound to the upper 

 surface of the prolonged cartilage. Its proximal end reaches for* 

 ward so as to overlap somewhat the last but one of the ossified verte- 

 bral centra. The distal end appears to be embraced by the lateral 

 halves of the hindermost epural fin-ray. 



2. THE MINUTE STRUCTURE OF THE VERTEBRAL COLUMN. 



Kolliker has dealt with the microscopical structure of the termi- 

 nal portion of the tail. Schmidt has also described and illustrated 

 with a number of figures some points in the finer structure of the 

 vertebral column. 



In all the vertebras of this fish the bases of the arches, upper 

 -and lower, come into contact with the sheath of the notochord. \ In 

 my specimen, 12. 5 cm. long, these bases consist of clear cartilage, but 

 at the inner ends of the bases small portions of the cartilage are ab- 

 sorbed. This process of absorption continues as the individual 

 increases in size, until probably the greater part of the cartilage is 

 removed. While this is going on, trabeculae of bone are depos- 

 ited, which look exactly like the bone deposited elsewhere in the cen- 

 trum; but the crust of bone which first spread over the base of the 

 arches serves to define their boundaries. See figures i, 5, 6, 7. 



It is interesting to observe that in the case of the simple verte- 

 brae, as those, of the dorsal region, the cartilage contained in the 

 upper arch is segmented off at the surface of the centrum from the 

 cartilage enclosed within the centrum; while the cartilage of the 

 arches in the middle of the tail is continuous with that of the cen- 

 trum. 



The cartilages which have already been referred to as appearing 

 in pairs on the underside of each dorsal vertebra, may be now more 

 fully described. They are shown in figures i, 5, 6. As seen in 

 longitudinal section, they may be regarded as thin trigtilanar plates, 



