EASTMAN: MYLOSTOMID DENTITION. 219 



spect to each other, and the two right-hand plates have become dis- 

 arranged. 



3. None of tlie palatal plates retain their natural position with 

 respect to one another. 



By orienting the two right-hand plates in corresponding fashion to 

 the two left-hand, and then approximating the disjoined halves of the 

 dentition, a symmetrical pattern is formed, such as is shown in Figure C, 

 and repeated in Plate 1. An obvious feature of this arrangement is that 

 the only elongate linear margin of any of the plates is directed parallel to 

 the median line, and it is along this line that the paired elements 

 having a single straight margin are mutually in contact. The evidence 

 of co-adaptation presented in this respect is sufficiently striking, and it 

 will be at once recalled that analogous conditions are found in various 

 types of Dipnoan dentition. 



We have now to apply the most crucial test open to us with the 

 means at our command. It consists in bringing the functional surfaces 

 of mandibular and palatal plates together and observing the extent of 

 their mutual correspondence. Immediately this is done it becomes 

 patent that a close coincidence exists between the contour lines of upper 

 and lower dentition ; the jaw-parts fit together as accurately as may be 

 when the mouth is closed, and their impact is exactly such as is capable 

 of producing the observed marks of wear. The triturating areas that 

 were left uncovered in the preceding restoration (Fig. B) are now 

 brought into much closer adjustment above and below, the contrast 

 presented by Figure C in this respect being self-evident. It thus appears 

 that the fourth in the list of objections to Dean's arrangement {v. p. 217) 

 is inapplicable in the present instance. 



Amongst other constant features of the anterior pair of palatal plates 

 are to be noted the following : The linear inner (ental) margin is 

 elevated into a distinct ridge, which increases in height and breadth in 

 the vicinity of the antero-internal angle of the plate, where it shows 

 evidence of attrition. The marks of wear extend not only along the 

 side of this ridge, but also over the concave surface of the plate immedi- 

 ately adjacent to it. The summit of the elongate, mesially placed 

 tubercle is worn, and the sunken area in advance of this, extending as 

 far as the antero-external angle is the most conspicuously worn of all. 

 This concavity is particularly well displayed in the detached specimens 

 figured by the writer in Bulletin Mus. Comp. Zool., 50, no. 1, Plate 1, 

 Figures 2, 3. Now, the significance of all these features becomes 

 apparent when the functional surface of the plate in question is applied 



