froin the Marl- Slate of Middtrichje. 53 



the sides being unequal, particularly the scoop-like portion, 

 one side of which is more developed than the other. And the 

 root likewise is turned a little to one side, especially in the 

 second lateral. 



The secondary or pctalodontoid teeth are not more than 

 I in. long, and about the same wide ; they are depressed and 

 partake otherwise of the general characters of the primary 

 teeth. They are more inequilateral and oblique than the 

 smaller primaries, one side being much more arched than the 

 other. The cutting-margin is slightly arched and denticu- 

 lated, but is narrow and only a little concave ; the cru.^h- 

 ing-disk, too, is wider than long, the transverse, imbricated 

 ridges being reduced to about half a dozen. 



The upper surface of all the teeth, whether primary or 

 secondary, is covered with a thick layer of o])aque white ena- 

 mel-like matter. This has a very striking appearance, con- 

 trasting as it does with the dark hue of the rest of the tooth, 

 and being strongly defined around the margin by a thickened 

 rim, which is best seen when the tooth is turned with its face 

 downwards. 



And, moreover, when the enamelled surface is a little worn, 

 it becomes pitted and freckled all over with dark irregular 

 points, which are sometimes elongated, particularly on the 

 anterior or cutting-margin. 



There is little difficulty in determining the manner in which 

 these curious teeth are placed in, or rather on, the jaws ; for 

 apparently the whole of the teeth of both jaws have been 

 found lying in their original position, though the jaws them- 

 selves have entirely disappeared, they having undoubtedly 

 been composed of cartilage. Ha\'ing carefully examined Mr. 

 DufTs specimens, which Avill shortly be described, and after a 

 full consideration of Count Miinster's figures and descriptions, 

 we can have little hesitation in giving the following account of 

 the an-angement of these rather extraordinary dental organs — 

 and this notwitlistanding that we are acquainted with no- 

 thing exactly like it, either in fossil or recent fishes, except 

 in the so-called Climaxodus. 



First, then, the teeth are arranged in both upper and lower 

 jaws (PI. II. figs, 2, 3) in precisely the same order. In both 

 they are placed in transverse horizontal rows, across the ante- 

 rior portion of the jaws, and in such a manner that never more 

 than a single row in each jaw is in operation at the same time. 

 Each such horizontal row is composed of seven teeth (five 

 primary, two secondary-), placed lengthwise, Avith the cutting- 

 margin in front. A large symmetrical primary tooth is situ- 

 ated on the longitudinal median line, or exactly "ver the svm- 



