﻿210 ANTIA.RCIIA. 



the shield, except the postero-lateral pair, are firmly fixed together by 

 sutures. There is a crown-shaped median occipital (m.occ), 

 bounded upon either side by a somewhat smaller lateral occipital 

 (l.occ.), and separated from the great opening in front by a narrow, 

 transversely elongated, postmedian plate (pt.m.). A very small, 

 approximately quadrate angular plate (ag.) adjoins the outer 

 margin of the lateral occipital on each side ; and a long, narrow, 

 lateral element (I) extends on each side of the median opening from 

 the front margin of these plates continuously to the rostral border 

 of the shield. A large single premedian plate (p.m.) is interposed 

 between the anterior extremities of these laterals, forming both the 

 front border of the median opening and the extremity of the snout. 

 The narrow space on each side, between the lateral and angular 

 plates and the anterior border of the armour of the trunk, is filled 

 by a loose extra-lateral plate (e.l.), which seems to have formed the 

 operculum ; its posterior margin was evidently free, but its anterior 

 strongly convex margin is notched in such a manner as to suggest 

 the ordinary articulation of a fish-operculum (see PL V. fig. 6). 

 The orbits seem to have occupied the rounded extremities of the 

 great median opening, these being separated by a thick, loose, 

 quadrate plate, with laterally produced hinder angles, well shown 

 from the inner aspect in PL V. fig. 2, p; this element (the " os 

 dubium " of Pander) is ornamented externally, but exhibits a deep 

 pit in the middle of its inner face, evidently for the reception of the 

 pineal body, and it may thus be known as the pineal plate. 

 Immediately in |ront of the latter there seems to be a thin, narrow 

 bone (see No. 19804 a, p. 222), but this has not yet been clearly 

 observed. In the position of the orbits themselves, a thin, oval, 

 convex or concave, smooth plate is often observed ( PL V. fig. 2, o), 

 and this may probably be interpreted as an ossification in the 

 sclerotic. 



The sensory canals upon the cranial shield are nearly parallel 

 with its border, one directly crossing the median occipital plate 

 transversely, another similarly crossing the premedian, and a lateral 

 pair extending along the long axis of the laterals. These and the 

 transverse hinder canal meet in an angulation on the lateral 

 occipitals, whence also a branch runs along the dorso-lateral plates 

 of the trunk, forming the " lateral line." 



The head seems to have been movably articulated with the 

 trunk, out not by any ginglyraoid processes or surfaces. The dermal 

 armature apparently extends over the whole of the abdominal 

 region, but does not include the anus. Its ventral surface is 

 flattened, while the dorsal shield is much arched ; and all the 

 plates are deeply overlapping. There are two median dorsal 



