The Structure of Dinichthys 159 



bone from the outside (B) and inside (A). In Dinichthys the complete ring is seldom found. 

 It is known from a specimen of Dmichthys magnijicus Huss. & Br. in the Buffalo Museum. 

 (Hussakof and Bryant, 1918; p. 41, fig. 11). Another specimen is preserved in the 

 American Museum of Natural History. A third is figured by Newberry (1889). 



Text-figures 42 and 43 show the total reconstruction of the head of Dinichthys inter- 

 medius Nwb. from the side and from above. From them we see that the proposed recon- 

 structions are distinct from all that have been made heretofore. The head is much more 

 compact and rounded in all directions; no sharp angles or 

 processes protrude and there are no large spaces between 

 the single plates. The head is sharply curved and thus 

 the side and front margins of the head roof, SO and PSO, 

 become nearly vertical. The lower jaw has at last 

 obtained a distinct position. The nasal openings are 

 also fixed. ° ' ' ^ f-^ 



I may mention here that the well-known "eye balls," , Text-figure 41. 



1 -111 XT 1 /-,r,r,r^\ ■ 1 • 1 r. A piecc of the sclerotic ring in 



described by Newberry (1889) must, in accord with Stet- ^^^ hth ■ A { ' d ■ B f 



son's investigation (1930), be regarded as nasal capsules. outside. 



Probably they were placed behind the nasal openings, 



under the plate PrO. There is nothing to say about the sensory canals. I can only note 



that the short canal on P?v( unites the canal on PrO with that on SO, so that each eye 



opening is nearly encircled by a sensory canal. 



Finally, it can be pointed out, that the division of the head of Dinichthys into two 



regions, the head roof and the side plates, is quite like the division of the head in all the 



fishes into the brain case and the visceral region. The head roof in Dmichthys, like the 



brain case in the fishes, serves to protect the brain, eyes, ears and olfactory organ. The 



side plates like the visceral skeleton form the upper and lower jaws. It is difficult to say 



if we have here only analogy, or also homology. 



THE BODY CARAPACE 



The body carapace surrounds the front part of the body as a continuous ring. It is 

 broad in the dorsal part, becomes narrower downwards, and is narrowest where the 

 dorsal and ventral parts meet. The latter is very large and forms a flat, long "plastron" 

 which protects the ventral side of the fish. The whole armor is built from nineteen plates, 

 which are more or less strongly connected. We find here, as before in the head shield, 

 exclusively squamosal sutures. The single plates overlap each other very strongly and on 

 each plate we find clearly limited overlapping margins. Some plates {PL, Sp) in addition 

 to the sutures, are connected with the help of long thin spines, which go deep into the 

 neighboring plates. In spite of these solid junctions, the carapace plates were not as 

 strongly connected with each other as were the plates on the head roof, and almost without 

 exception they are found isolated. 



