EASTMAN : MYLOSTOMID DENTITION. 217 



cide at all points we have evidently found the true arrangement ; since 

 only in this manner could the parts have interacted during life. Appli- 

 cation of this test to the single individual of Mylostoma we are 

 considering, and also to an extended sei'ies of detached plates, shows 

 that Dean's reconstruction fails to explain all the facts, and only ex- 

 plains some of them by positing anomalous, and pro tanto improbable 

 conditions. In a word, tlie arrangement is unable to satisfy the test of 

 totality. It is, therefore, inadequate, and the initial assumption upon 

 which it is based must be regarded as erroneous. 



The principal objections to Dean's reconstruction may be thus 

 summarized : — 



1. The proposed arrangement necessitates the assumption of jaw- 

 movements in Arthrodires which are unparalleled amongst Chordates. 



2. No close analogy is suggested by this arrangement with the 

 dentition of related forms. 



3. Some conspicuous indications of wear, the position of which is 

 constant in all plates thus far brought to light, are wholly unaccounted 

 for by this arrangement. 



4. According to this arrangement, the marginal contours of upper 

 and lower dental plates do not coincide. A considerable portion of the 

 oral surface of all the plates is left uncovered when the jaws are closed, 

 even including areas which show indications of wear, 



5. One of the two palatal plates found lying in apposition in the 

 nearly complete example of Mylostoma (the one called " premaxillary " 

 by Dean) is observed to present a worn surface immediately adjoining 

 the contact margin with the so-called " maxillary " element. The latter, 

 however, is unworn along the contact margin, but is raised there into 

 a prominent ridge. Supposing these plates to have been naturally in 

 contact, they must needs exhibit similar evidence of attrition along 

 their common margin ; since they do not, they must have been arranged 

 in some other manner. 



6. The only strictly linear margins of any of the plates are not in 

 contact with each other, nor are the members of either pair in direct 

 apposition along the median line. 



Eastman's Interpretation. — The arrangement proposed by the 

 present writer was first established upon the evidence of detached plates, 

 which were fitted together conforraablv to the marginal contours of 



O I/O 



upper and lower dentition, and in such manner as to account at all 

 points for reciprocal marks of wear. Its efficiency was afterwards tested 

 by applying the same arrangement to the single example of Mylostoma 



