Dr Traquair on the Genus Gyracanthus (Agassiz). 99 



we take first one of tlie triangular series, it may easily be seen 

 that the two walls, or laminae of which the bone is composed, 

 are united at the apex and along the two thick sides which 

 meet at the apex, but that they are separate at the thin base, 

 at which accordingly the cavity was open. It may also be 

 seen that the edges of the basal opening do not coincide, as 

 careful development of these edges shows that the one on the 

 convex side of the bone is indented by a large angular notch 

 or sinus, which runs up for some distance in the direction of 

 the apex ; this appearance I have seen in every case in which 

 I have looked for it. The internal cavity is at once distin- 

 guishable, filled with matrix, when a specimen is broken or 

 cut across. I have equally assured myself of the hollow 

 character of the bones of the second series. 



If we now look at the texture of these bodies we shall be 

 at a loss to explain the expressions " imperfectly ossified " 

 and " semicartilaginous," used by Messrs Hancock and 

 Atthey in the passages already quoted. On examining the 

 surface with a lens its apparent fibrous aspect is seen to be 

 due to its being closely covered with minute gTOoves inter- 

 spersed with small openings, these markings being clearly 

 vascular in their nature and of the same essential character 

 as those on the inserted portion of a Selachian spine, only not 

 so regularly parallel as is usually the case in the latter. On 

 making microscopic sections, transverse and longitudinal, 

 through the substance of the supposed " carpal bone," it is 

 found to be completely traversed by a close network of 

 vascular or Haversian canals, the canals in some parts en- 

 larging so as to give a rather more open character to the 

 tissue than is found in the internal part of a GyracantJms- 

 spine itself, while the ground-substance, hard and calcareous, 

 is permeated by minute branching and anastomosing tubules, 

 which are frequently seen to radiate from the vascular canals. 

 This is not, however, the structure which Selachian cartilage 

 assumes when calcified or "ossified";^ on the contrary, if 

 the tissue be not vascular dentine, it is certainly very like it. 



1 For an account of tlie structure of calcified Selachian cartilage, see 

 Williamson on the "Structure and Development of the Scales and Bones of 

 Fishes" (Phil. Trans., 1851). 



