XX ANTIARCHI 533 
portion and a much larger hinder part investing the greater part 
of the trunk, both of which are strongly arched above and 
flattened ventrally, with a movable articulation between the two. 
The cephalic shield is formed by numerous symmetrically-dis- 
posed tuberculated plates, suturally connected with one another, 
and, like the other exoskeletal structures, containing bone 
Hy 
mT AY 
TH ROY 
mmm te LECNUO COE 
Fic. 322.—Restored outline of Pterichthys milleri. The upper figure represents a dorsal 
view, and the lower a lateral view. The dotted lines indicate the course of the 
lateral line system. a.d.J, Antero-dorso-lateral; ag, angular ; a.m.d, anterior 
median dorsal ; a.v./, anterior ventro-lateral ; ¢./, extra-lateral or operculum ; /, 
lateral ; Z.occ, lateral occipital ; m, median or interorbital plate ; m.occ, median 
occipital ; 0, orbit ; p.d./, posterior dorso-lateral ; p.m, pre-median ; p.m.d, posterior 
median dorsal ; pt.m, post-median ; p.v.l, posterior ventro-lateral. ~—, Plates 
investing the limbs: c, central ; d.a, dorsal anconeal ; d.ar, dorsal articular ; e.m, 
external marginal ; 7.m, internal marginal; m.m, marginals; ¢, terminal. (From 
Traquair.) 
lacunae (Fig. 322)." The orbits are close together, near the 
middle of the dorsal surface, and between them there is a small 
median interorbital plate, with a deep pit on its inner surface, 
possibly for a parietal organ. A small lateral plate (¢./.), evidently 
free behind, suggests the presence of an operculum. Nothing 
is certainly known about the jaws or the nostrils. The mouth 
is situated just behind the anterior margin of the cephalic shield 
on the ventral surface, and in front of it there are two plates, 
1 Traquair, dnn. Nat. Hist. (6), ii. 1888, p. 485. 
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