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Bashford Dean Memorial Volume 



new plates in this part of the head. In the majority of the species these side plates are 

 generally found isolated, only very seldom are they attached to the head roof or to one 

 another. Some of them are small, others probably do not ossify in all species — not even 

 in all individuals. As a result of this the arrangement and relation of the side plates is the 

 most imperfectly known and the most disputable question in the head structure of the 

 Arthrodira. 



In this paper I shall first describe the single plates and later try to give a picture of 

 their reciprocal positions and the relation to the head roof. At present we know only 

 seven pairs of these plates. Five pairs belong to the upper jaw, and only two to the lower. 



Text-figure 21. 



The right sub-orbital plate of Dmichthys intermedius Nwb. seen from the outside. The inset portrays a 



cross-section of the "handle" in the line a-b of Text-figure 23 I and II. 



A, real handle; a-b, line of section shown in mset; B, tongue-shaped part of handle; c-d, line of section shown in inset of Text-figure 



23; gr, groove on underside of tongue-shaped part; I/, lower hind corner of SO: of, upper hind corner of SO; oh, upper front 



corner of SO; Or, orbit; pn, contact point with post-nasal plate; pro, contact point with pre-orbital plate; R^ and R4, ridges on 



inside of SO; sp, split between real handle A and tongue-shaped part B; IX and X, sub-post-orbital canal; XI, gnathal canal. 



The upper jaw plates are: the sub'orbital, post'nasal, post'sub-orbital, postero'supra- 

 gnathal and antero-supra-gnathal. Those of the lower jaw are : the infra-gnathal and post- 

 infra'gnathal. 



Sub-Orbital. — This is a large, plain plate (Text-figures 21, 22 and 23) which covers 

 nearly the whole lateral part of the head and forms the lower margin of the orbital opening. 

 Its outline reminds one of a cleaver, with a big, flat "blade" with the edge turned upward, 

 and with a short ''handle." The rounded incision between the blade and handle forms 

 the margin of the orbit. The blade composes the largest part of SO. It is flat, oblong, 



