1870.] 11 [Blis3. 



the fin. A prolongation of the upper anterior edge of the sides of 

 the boat extends downward to the keel below the middle of the 

 length of the oval opening. The anterior portion of the keel is 

 somewhat prolonged, and fits into an opening in the occiput, by 

 which additional support is afforded to the apparatus. 



This naviculate bone bears three spines; two of them, one imme- 

 diately succeeding the other, are peculiarly articulated with the bone 

 which supports them. The third spine is placed at the hinder ex- 

 tremity of the boat, on a bone, which, though soldered to the bone 

 bearing the two anterior spines, is yet distinct from it, as is shown 

 by the direction of its fibres. A tendon, the use of which will be 

 described presently, extends from the upper part of this third spine 

 to the base of the second. 



The first spine is stout and roughened on its anterior face with 

 small, blunt teeth. The posterior face of the spine is deeply grooved, 

 and the two edges of the groove somewhat approach each other to 

 form a kind of shoulder, which, when the spine is erect, rests upon a 

 projection on the anterior face of the second spine. The portion of 

 the first spine which rests upon the thickened prow of the boat ex- 

 pands on each side, and the bases of these expansions are formed 

 into articulating surfaces, and rest upon corresponding facets on the 

 bone beneath. Between the articulating facets of the naviculate 

 bone, rises a little crest, hollowed so deeply on each side as to form a 

 complete perforation. Into these hollows project two inward prolon- 

 gations of the lateral articulating arms of the spine. But these in- 

 ward prolongations of the arms do not meet through the perforated 

 crest. It is, as will be seen, an approach to the linked j oint mode of 

 articulation described in Doras. 



The second spine of the fin, which is smaller than the first, is 

 forked at its base, and bestrides a small crest formed by an upward 

 extension of the keel of the boat. The two arms of the spine are 

 prolonged downwardly, as previously stated, through the oval open- 

 ing in the sides of the boat, and are supported movably at the point 

 where the bony rod from the anterior edge of the boat is joined to 

 the keel, below the opening in the side of the boat. The anterior 

 face of the spine is provided with a protuberance just above the 

 fork, upon which rests the shoulder of the large spine in front. 



In Batistes, as in Doras, the large spine is used as a weapon, and 

 like that of Doras cannot be lowered till the other spines are' moved. 

 When the spines are raised the second spine rides forward over its 



