of Gei'tain Body-plates in the Dinichthyids. 49 



postero-lateral margin of the median dorsal, and the curve of 

 the overlapped margin on the lower portion of the plate under 

 discussion will fall together, on superimposing the one upon 

 the other. The prolongation of this curve is observable on 

 the inner margin of the suprascapula, where the same is over- 

 lapped by the median dorsal. In other words, the superin- 

 cumbent margin of the median dorsal leaves an impressed 

 area on the antero- and postero-dorso-laterals, by means of 

 which the relative positions of these three plates can be 

 accurately determined. 



On examining the postero-dorso-lateral, it will be noticed 

 that the curve formed by the overlapping margin of the 

 median dorsal is intersected nearly at right angles by another, 

 longer, more ragged, and more deeply incised contour-line 

 than the first ; this marks the posterior boundary of the antero- 

 dorso-lateral. The sensory canal system of the latter plate is 

 continued on to the postero-dorso-lateral, sometimes as a single 

 and sometimes as a double furrow, and furnishes another con- 

 venient clue to the orientation. The indented area already 

 referred to on the so-called " free margin " of the plate was 

 formed by the postero-lateral ; and it is possible to reconstruct 

 the form of the laterals from their imprints upon the plates 

 annectant to them above and below, their curvature being 

 indicated by the clavicles. It is to be hoped that by this 

 means some of the unidentified plates existing in various 

 museums may be recognized as the missing laterals. 



From the above it will be seen that every plate present in 

 the body armor of Coccosteus has its representative in Dinich- 

 thys^ and that the conditions of overlap and underlap are the 

 same in both forms. 



Median dorsal plate {'' Dorsal shield^"^). — The configura- 

 tion of the posterior ridge on the under surface of this plate is 

 well shown in a number of specimens belonging to the Terrell 

 and Allen collections, two of which show that its inferior 

 extension is greater than has been supposed. A preparation 

 of these particular specimens resulted in the exposure of a 

 deep spoon-shaped cavity at the extreme end of the terminal 

 process on its dorsal aspect. The concavity is slightly emargi- 

 nate below, and is continued up the ventral side of the process 

 in the form of two broad, shallow depressions, which rapidly 

 become narrower and deeper, so as to form well-marked chan- 

 nels. These gradually disappear, however, after traversing the 

 lowermost portion of the inferior keel for a considerable dis- 

 tance. Corresponding to the ventral furrows are two dorsal, 

 which extend from the bowl of the spoon along the ' handle' 

 as far as the posterior margin of the dorsal shield proper. 

 Here the terminal process broadens out on either side, and its 

 roughened or striated upper surface evidently served for mus- 



Am, Jour. Sci.— Fourth Series, Vol. II, No. 1. — July, 1896. 

 4 



