ON JANASSA BITUMINOSA. 349 



result of wear, for they exhibit on their surfaces the internal struc- 

 ture of the matter composing the tooth. And that the opposing 

 crushing-disk is not equally and mutually worn arises from the 

 fact that it is covered with a layer of hard enamel-like matter. 



The existence of the transverse grooves would seem also to 

 prove that while they were produced by the rubbing-motion of 

 the teeth upon each other, the motion itself must have been very 

 limited, or neither the grooves nor the sharp definition of the 

 facets could have existed. And in this way we have a corrobo- 

 ration that the retention of the old effete teeth is merely for the 

 mechanical support that they supply to the upper row of teeth, 

 upon which teeth alone devolves the function of cutting and 

 crushing the food. 



The four groups of teeth obtained by Mr. Duff at Midderidge 

 are very instructive, and though in a more or less disturbed 

 state, are quite sufficient to show the original disposition in the 

 mouth. One of the specimens was quite perfect when found ; 

 but unfortunately an idle lad got hold of it, after the quarryman 

 had carefully laid it aside, and in the mere lack of thought broke 

 away a great number of the teeth. Happily however the ante- 

 rior portions of nearly the whole of them are still left sticking in 

 the matrix, so that not only their number can be ascertained, 

 but likewise the exact limit of those belonging to the upper and 

 lower jaws respectively, and their precise arrangement thereon. 



This specimen of the buccal armature was not only complete 

 when deposited, but is lying on the slab in its natural position ; 

 and probably when buried the whole fish was present, and lay 

 with its back uppermost. Consequently, the mouth being situ- 

 ated beneath, as in the Sharks and Bays, the teeth of the over- 

 hanging upper jaw would lie in advance of those of the lower. 

 Such is the case in the specimen now before us, as is determined 

 by the presence of a quantity of shagreen, indicating as it does 

 the direction in which the body of the fish was deposited. There 

 are about three inches of this shagreen, extending from the pos- 

 terior margin of the mass of teeth, or those which belong to the 

 under jaw. And, in fact, there can be little doubt, that had the 

 slab been continued backwards sufficiently far, we should have 



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