NATURAL HISTORY SOCIETY OF GLASGOW. 339 



of life, put in play to form a basement for a polyzoon, had to 

 depart from the well-established mode, in order to produce a 

 projection beyond the surface line of a living encrinite. A little 

 light is shed on the phenomenon by observing that on the upper 

 side of the tubercle, and there only, a part of the stem exists, as 

 seen in the other examples. 



Petalodus Rastingsiae, Owen. 



In forming a polished cross-section of the tooth of Petalodus 

 Hastingsiae, characters were observed of much interest, tending to 

 give edge and strength, — much in the manner of the blacksmith 

 inserting a piece of steel between plies of iron in the axe to admit 

 of a fine edge, which the iron incasement tends to prevent from 

 petty fractures or destruction. This tooth, in section, is conical, 

 half as broad at the base of the crown as the entire length. The 

 whole crown is covered with enamel not thicker than the eda-e of 

 a sheet of note paper, and apparently this delicate covering meets 

 so as to form the cutting edge. But a mid-bar, apparently of 

 enamel, forms the true cutting edge, which passes centrically 

 between them for nearly half the length of the crown. About 

 one-third of the length of the tooth from the cutting edge is white 

 (bone-like, with the mid-bar distinct), the rest dark within the 

 enamel. The white part has, on each side of the mid-bar, delicate 

 lines passing between it and the outer enamel. The cutting edge 

 is produced, technically, by a short cannel or abrupt finish. 



These remarks probably comprehend all the particulars of 

 structure as seen in section, and inferences. But a few words 

 more may be said in reference to M'Coy's description of this tooth 

 (Brit. Pal. Fossils, page 635). He states that " the cutting edge 

 alone being marked with a row of punctures (twelve in one line) ; 

 when worn, however, a fine line extends from each of these 

 punctures half way down the crown, producing a structure scarcely 

 visible to the naked eye." Now this description, true to the facts 

 observed by him, admits of explanation. Between the external 

 enamel and the mid-bar described above, which forms the cutting 

 edge, numerous partitions of bone, with equally numerous inter- 

 spaces, are easily discerned on each side of the cutting edge, and 

 these extend as far down the tooth as the mid-bar reaches. An 

 interpreter is needed at this point. This fish sharpened its own 

 teeth, and it was easier and more fit to grind a partitioned 



