174 



Bashford Dean Memorial Volume 



My opinion is that these two plates are always represented as two independent 

 elements. I have never found them fused, either in specimens in the American Museum 

 collection, the collection in the Buffalo Museum, or that in the Museum of Comparative 

 Zoology at Harvard. On the contrary, the isolated plates were frequently found with 

 very clearly overlapping margins. If these plates really could fuse, such was probably 



Text-figure 

 The antero-median-ventral plate : A, from outside; B, 'from inside. 



a-b, longitudinal section of front part of plate in line a-b of A; T, the T-shaped thickening. 



only the case in aged individuals. This contact would then have been of a secondary char- 

 acter. These two plates in all cases have preserved separate ossification centers and in- 

 dependent series of ossification rays, and cannot, therefore, be regarded as a single plate. 



THE CONNECTING PLATES 

 BETWEEN THE DORSAL AND THE VENTRAL SHIELDS 



As in all other Arthrodira, the dorsal and ventral shields in Dmichthys are connected. 

 Without a connection — as, for example, in Dean's reconstruction — the whole structure of 

 the body carapace becomes too unsolid, weak and unsatisfactory as an attachment for the 

 strong body muscles. The dorsal and ventral shields are connected by the help of two 

 plates, the spinal and the intero-lateral. Thus in this character also Dinichthys cor- 

 responds with all the other Arthrodira. As we know, since Newberry's time these two 

 plates have been counted as portions of the clavicular. Nobody has determined and 

 described any of them before, in spite of the fact that Claypole as early as 1893 pointed 

 out the limit between Sp and AL, and that the collection of the American Museum con- 

 tains well preserved specimens of both plates. 



