On the Structurp and 119 



have formed the apex of the arch of the lower jaw — a con- 

 dition of things scarcely comprehensible. 



If now we transfer this spine to the position of the post-dorsal 

 fin, and bury it in the soft parts all except the denticles, the 

 elongation backward by the successive addition of sheaths and 

 denticles becomes intelligible and natui-al. 



There are some anomalous features in this fossil which re- 

 quire notice, viz., there is no distinct line of demarcation be- 

 tween an exposed and a buried portion, such as we find in most 

 of the defensive spines of sharks, unless, as seems probable, all 

 the shaft was buried, and only the denticles exposed. Another 

 peculiarity is the absence of the medullary cavity found in most 

 dorsal spines of sharks. This is quite conspicuous in the spines 

 of Hyhodus, Ctenacanthus, etc.: but in the rays the spines 

 are solid, and there is little distinction between the exposed 

 and buried parts. The exceptional characters just mentioned 

 need not therefore, be considered incompatible with the view 

 that these fossils are spines. 



The segmented structure of Edestus is its most marked and 

 anomalous feature, but one equally so whether it be considered 

 spine, jaw, rostrum or intermandibular arch. It is undoubted- 

 ly to this structure that we must ascribe the absence of a large 

 medullary cavity, as each segment bearing a denticle seems to 

 have been nourished independently of its fellows. It is also ap- 

 parent that the growth of this organ was by additions to the 

 summit of successive sheaths, each of which carried a denticle. 

 This is strikingly different from the mode of growth of all 

 sharks' spines known, as these increase by additions to the base, 

 and are thus pushed upward and lengthened. The same is true 

 of all rostra which are used as weapons of defense or offense. If 

 we consider the segments of Edestus as homologues of a dental 

 series, we encounter the same difficulty. A row of teeth of 

 Orodus, for example, which consist of enameled crowns with 

 flattened bony bases lying in contact and compressed together, 

 considered as a whole presents considerable analogy with our fos- 

 sils ; but there too the growth is from behind forward, new 

 teeth moving up to take the places of such as are broken or worn 

 away. The numerous disconnected segments of Edestus Hein- 

 richsii furnished me by Mr. Butters seem to prove conclusively 

 that the spine was elongated by the addition of a sheath carry- 

 ng a denticle to the extremity and under-side of the pre-existing 



