192 Bashford Dean Memorial Volume 



But to reconstruct their position and function is a hopeless task, in our present stage of 

 knowledge. 



THE COMPLETE RECONSTRUCTION OF THE HEAD 

 AND BODY CARAPACE OF DIHICHTHTS 



After the detailed description given of the head and body plates in Dinichthys, it is 

 not necessary to spend much time in describing the complete armor. This has already 

 been done in the preceding sections. I shall here only summarize the new points in my 

 reconstruction (Text'figures 79, 80 and 81) as follows: 



1. The head is strongly curved from side to side and from back to front. 



2. The fossa condyli has a horizontal position. 



3. Four new plates are found in the head: (a) post-nasal, limiting the nasal 

 openings; (b) post-marginal; (c) post-sub-orbital, serving as an attachment for the 

 post-infero-gnathal; (d) post-infero-gnathal, which comes in contact with post-sub-orbital. 



4. The gnathal elements are arranged so that the position of the cutting edges is 

 strongly vertical. 



5. This causes the lower front corners of right and left IG to come nearer together. 



6. The body carapace is narrower and higher than proposed before. 



7. The dorsal and ventral shields are connected. 



8. We can assert the presence of the following plates: (a) postero-lateral; (b) 

 spinal; (c) antero-lateral. 



9. The two last plates serve to connect the dorsal and ventral shields. 



10. The median plate in the ventral 'carapace is not single but double (AMV 

 and MV). 



OTHER STRUCTURES AND ORGANS OF DIHICHTHTS 



About the inner skeleton of Dinichthys we know very little. Practically nothing of 

 this is preserved in the fossils. Some conclusions can be drawn from impressions on the 

 armor, from homology with other animals and from logical necessity. Of course, all these 

 arguments are of a more or less subjective character and can, in no case, give exact results. 



THE PRIMORDIAL NEUROCRANIUM 



In the Arthrodira the primordial neurocranium was built of cartilage, and no traces 

 of it have ever been found. The only things left to direct us are the impressions and 

 processes on the under side of the head roof. These characters give no positive facts, and 

 the significance of the different impressions can be explained in various ways. 



The first author to mention the position and shape of the neurocranium in the Arthro- 

 dira was Woodward in describing Homostius (1891.3) and Dinichthys (1922). According 

 to him: "The bony cranial shield extends backwards beyond the brain, and there can be 

 no doubt that in all Arthrodira it covers the branchial chambers behind the occiput." In 

 Dinichthys the thickened part of the central plate (Text-figures 13 CR) is the "ossified 

 cartilages on the posterior end of the brain-case." The ridges of the lateral consolidated 

 part (Text-figure 13 LCP) are the "ossified upper part of the lateral walls of the brain- 



