298 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
fused with the centrum below and protecting the terminal filament. 
Lotz terms these the ‘ grosse Deckstiicke,’ 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- 
bre ; 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 toa 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 vertebre. 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. i 
To recapitulate, then, the homologies of the modified ventral parts 
of these posterior vertebre : Zhe 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 wpper arches of 
which have disappeared. The four succeeding centra and their wpper 
arches have become aborted, leaving only the hemal 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 
vertebree, 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 
