The Structure of Dimchthys 



169 



Ag., for example, the wing on AL is always clearly shown. Traquair (1890.4) described 

 it as part of IL. This partis developed especially concisely in Coccosteus canadensis Wd. 

 which I had the privilege of studying in the Buffalo Museum. In some of the Wildungen 

 Arthrodira also, AL shows the same structure. In Acanthaspida it is also quite easy to see 

 traces of the same structure (Heintz, 1929.1; Figs. 8, 9, 12, 13, and 14). The wing of AL 

 is described as part of IL. Finally, in Homostius (Heintz, 1928) the structure of AL, in 

 spite of a very high degree of specialization, shows traces of the same characteristic 



outlines. 



PosTERO'LATERAL.—This plate (Text-figures 54 and 55 PL) has never before been 

 described in Dmichthys, In the majority of reconstructions it is absent (Newberry, East- 

 man, Dean, Hussakof, Branson, Bryant, Stetson). Only in the reconstruction exhibited in 

 the American Museum of Natural History do we find PL (made of plaster) as a relatively 

 large, broad plate. In Dean's description and photographs of this reconstruction of 

 Dimchthys (1909) this plate was not included. 



The present writer has been successful in find- 

 ing, first one quite good example of this plate in the 

 collection of the American Museum (Bungart coll., 

 1914; field No. 13), and later two perfectly preserved 

 plates in the Buffalo Museum (Nos. 15613/E3808 and 

 15616 E281 1). Text-figure 54 is a drawing of this plate 

 based on these three specimens. 



This is a relatively small, triangular plate, much 

 smaller than the one we find in the reconstruction in 

 the American Museum. It combines all three ele- 

 ments of the body carapace and is strongly connected 

 with them. On the outside, nearly the whole plate is 

 covered by the neighboring plates. Their overlapping 

 margins form the largest part of it. The not-over- 

 lapped part consists only of its thickened hind margin. 

 As we know, its spine-like upper corner extends into 

 the lower margin of PDL (Text-figure 54 PDLj. Its 

 side margin (Text-figure 54 ADL) comes in contact 

 with the previously mentioned impression on the over- 

 lapping margin on ADL. Lastly, the lower margin of 

 PL occupies the hind overlapping margin on AL. On 

 the inside, nearly the whole PL is covered by the 

 lower part of AL. 



To understand the structure of the dorsal shield Text-figure 54. 



of Dimchthys better, there is given in Text-figure 55 a ^^^ ^°"'^'°'^^'^^^^^^ '""" ^™"' '^' 

 contour drawing of the four side plates of Dinichthys (?) , , , , , r 



^ 1 1 r 1 ^' spine-like upper angle which tits into 



intermedius Nwb. connected together (from outside — cavity on pdl. 



