i8 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



The five hypural bones of this section are coossified to their re- 

 spective vertebrae in the same way as has been described in the case 

 of the vertebra immediately preceding. No cartilage is present, but 

 the lower surface of the vertebral centrum is composed of a thin 

 crust of bone, possibly of cartilaginous origin, and to this is coossi- 

 fied the corresponding hypural. The notochord enclosed by these cen- 

 tra has, on the underside, undergone little constriction. 



My longitudinal sections do not confirm Kolliker's view that the 

 next two hypurals are attached to the terminal centrum of the tail. 

 While the base of the first of these hypurals does come close up to the 

 hinder border of the last centrum, it is entirely distinct from the lat- 

 ter. At the same time its proximal end reaches backward and sup- 

 ports its own portion of the notochord. The next hypural is in its 

 turn brought into contact with the notochordal sheath still further 

 back, and is not at all coossified with the last developed centrum. 

 An examination of the next to the last hypural bone shows that its 

 base closely resembles that of the vertebrae in front; since there is, 

 next to the notochordal sheath, a layer of ossified cartilage. We 

 have therefore some reason to believe that the base of this hypural 

 has become coossified with the lower portion (hypocentrum) of an 

 imperfectly developed vertebral centrum. The base of the last 

 hypural is also somewhat expanded, and this expansion possibly rep- 

 resents another rudimentary centrum. It is not at all improbable that 

 at a later period of life these hypurals coossify by their proximal ends 

 jto the hindermost vertebral centrum, so as to produce the condition 

 described by Kolliker. 



It is interesting to note that the notochord over these two hypu- 

 rals just described, has suffered slight constriction, as if in sympathy 

 with the effort to develop additional centra. 



In my sagittal sections I find, in place of the staff-like bone 

 which is sometimes seen above the last described hypural, and which 

 is itself undoubtedly but another hypural, a row of three cartilages. 

 The two most anterior of these nodules of cartilage are covered with 

 a layer of bone. 



Turning our attention now to the band or rod of cartilage which 

 is prolonged beyond the ossified vertebral column and ends between 

 the lateral halves of the twenty-first caudal ray, we find that Kolli- 

 ker, as already stated, regards it as representing not only the upper 

 arches, but also the continuation of the vertebral centra. According 

 to Kolliker's description and figures, the cartilage under considera- 

 tion forms, for some distance beyond the last vertebral body, a com- 

 plete tube, which encloses the spinal cord, or its representative. On 



