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 



FIG. 322. Restored outline of Pterichthys miller i. 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.l, Antero - dorso - lateral ; </, angular ; a.m.d, anterior 

 median dorsal ; a.v.l, anterior ventro-lateral ; e.l, extra-lateral or operculum ; I, 

 lateral ; l.occ, lateral occipital ; m, median or interorbital plate ; m.occ, median 

 occipital ; o, orbit ; p.d.l, 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 auconeal ; d.ar, dorsal articular ; e.m, 

 external marginal ; i.m, internal marginal ; m.m, marginals ; t, terminal. (From 

 Traquair.) 



lacunae (Fig. 322). 1 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 (e.l.\ 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, Ann. Nat. Hist. (6), ii. 1888, p. 485. 



