NO. 1690. ON EDE8TUS AND RELATED GENERA— HAY. 59 



animals belonged among the Elasmobranchii ; (2) the organs that 

 represent to us these sharks were more or less imbedded in the soft 

 parts; (3) they must have been organs in the median plane of the 

 body; (4) a considerable part of each of these organs must have 

 been exposed externally — that is, they were not wholly buried in 

 the flesh. If these supposed dental masses w^ere in the mouth they 

 were consolidated teeth. The blades and the processes of enamel 

 descending from them correspond to the crown of the teeth, while 

 the shaft was formed through more or less complete fusion of the 

 roots of the teeth. Now, while the crowns of the teeth in Edestus 

 resemble closely those of some kinds of sharks, it must not be sup- 

 posed that the cutting edges and the denticles correspond to those 

 of sharks. Through strong compression of the teeth the original 

 cutting edges would have been brought to occupy what is now the 

 middle of the lateral faces of the teeth, while the anterior and pos- 

 terior midlines would have become the cutting edges. The denticles 

 of these edges were develo^Ded later and could not have been derived 

 from the original denticles. It will be seen, therefore, that the whole 

 tooth, if a tooth, suffered great transformation. 



If the organs under consideration w^ere not teeth they must have 

 been placed either in front of some of the median fins, like many 

 of the other ichthyodorulites, or possibly behind a dorsal fin, like 

 the stings of the Masticura, or on the back of the head, as the spine 

 of Xenacanthus. As regards Edestus^ it does not seem to be im- 

 portant whether the new segment of the compound spine, if spine it 

 was, came up before or behind the older ones, since probably the 

 whole shaft was buried in the flesh. If it came up behind the older 

 ones the spine might have been directed horizontally from the fin; 

 if the new^ tooth arose in front of the older ones the spine may have 

 been directed upward and backward in the fleshy front of the fin. 

 If in the case of HeUcoprion and Lissoprion the new tooth had arisen 

 behind the older ones the spiral would have been directed forward, 

 and on being subjected to oblique blows would have been liable to be 

 twisted from its socket. It seems almost certain, therefore, that the 

 new teeth came up in front of the older ones, in case, of course, the 

 organ belonged outside of the mouth. If this is true, the end that 

 has in this paper been called the front end is the hinder end and the 

 end called the hinder is the front end. 



The stings of the Masticura appear to be shed and replaced by new 

 ones. In Aetobatis there may be as many as five or six of these 

 spines present at once. The statements regarding the origin of the 

 new spines do not agree. Giinther " says that in the Trigonidae the 

 stings are shed from time to time and replaced by others growing 



"Study of Fishes, p. 342. 



