THE SKELETON OF THE VERTEBRATE HEAD. 105 



but as the two series are .attached, the one to the upper, the 

 other to the lower part of the bone, they form together a single 

 range for the support of the branchiostegal fold. 



I recognise a similar but more developed form of this 

 double arrangement of actinapophyses in the variously-modi- 

 fied cartilaginous or semi-osseous double styles or plates which 

 are attached to the convexities of the branchial arches for the 

 support of the respiratory membrane of osseous fishes. These 

 branchial actinapophyses also exhibit that jointed or multi- 

 articulate structure so generally presented by the rays of the 

 mesial and bilateral fins. 



This leads me to observe that I have not been able to 

 satisfy myself of the truth of the doctrine at present generally 

 held; that the inter-spinous bones and rays of the mesial fins 

 belong to the dermo-skeleton. I admit that, in certain in- 

 stances, these fins present more or less dermal bone in their 

 composition ; but I cannot see how fin-rays, from which the 

 skin and subcutaneous texture may be stripped, can be con- 

 sidered as portions of the dermo-skeleton. These rays can 

 scarcely, I conceive, be referred to the dermo-skeleton in the 

 cartilaginous fishes ; and as the rays of the bilateral fins re- 

 semble those of the mesial in their histological as well as in 

 their general relations, they ought to be placed in the same 

 category. The rays of the mesial, as well as of the bilateral 

 fins cannot, therefore, in my opinion, be consistently excluded 

 from that portion of the sclerome usually denominated neuro- 

 or endo-skeleton ; but like other elements of the endo-skeleton 

 which approach the dermal sclerous fibrous lamina, they may 

 coalesce with dermal bone. 



I have been led to consider the inter-spinous bones and 

 mesial fin-rays as actinapophyseal elements. With reference 

 to the mesial position and characters of these bones, I would 

 remark, that it appears to me to be quite permissible, on mor- 

 phological grounds, to look upon each inter-spinous bone, with 



