298 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



fused with the centrum below and protecting the terminal filament. 

 Lotz terms these the ' grosse Deckstticke,' and believes them to be 

 * Bogenstiicke des letzten Wirbels.' With this homology I cannot 

 agree, for two reasons. Firstly, the spinal chord does not stand in 

 the same relation to these bones as to the arches of the other verte- 

 brae ; it does not pass between them but lies in front (above) them 

 in the groove which they form ; posteriorly the rudiment of the ner- 

 vous tract is partly enclosed, but this arises from the upward growth of 

 the bone posteriorly, and does not correspond to a passage between 

 two arches. Secondly, these bones are not preformed in cartilage, as 

 their development shows, but are formed in membrane, thus belong- 

 ing to a different category to the arches, which are all preformed in 

 cartilage. These two facts appear to me to dispose of the ' Bogens- 

 tiicke' theory, and the question arises as to what is their true homo- 

 logy. They seem to correspond both in development and relations to 

 the dorsal longitudinal ridges of the vertebrae. They are direct con- 

 tinuations of these ridges which protect but do not surround the cen- 

 tral nervous system, and are developed by an ossification of mem- 

 brane. 



To recapitulate, then, the homologies of the modified ventral parts 

 of these posterior vertebrae : The free spinous process of the second 

 vertebra is the true spinous process of the arch of that centrum.. The 

 last centrum consists of three coalesced vertebra}, the upper arches of 

 which have disappeared. The four succeeding centra and their upper 

 arches have become aborted, leaving only the ha>mal arches to represent 

 them. The protecting bones on either side of the terminal filament of 

 the chorda are continuations of the dorsal longitudinal ridges of the 

 vertebra, and have no relations with the arches. 



VI.— THE DORSAL FIN. 



The dorsal fin adheres, to a certain extent, to the type of the im- 

 paired fins, consisting of fin-rays ossified in membrane, supported by 

 interspinalia, which are preformed in cartilage, but the anterior rays 

 and their interspinalia are modified for the formation of an organ of 

 defence capable of fixation in an erected condition. 



Anteriorly there is a small ossification lying in front of the large 

 plate for the support of the defensive spine, united to it by ligament 

 only and situated immediately below the skin. The plate with 



