210 
ANTI ARCH A. 
the shield, except the postero-lateral pair, are firmly fixed together by 
sutures. There is a crown-shaped median occipital ( m.occ .), 
hounded upon either side by a somewhat smaller lateral occipital 
(Z.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 ( l ) 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 
b y a loose extra-lateral plate (c.Z.), 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 PI. 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 PI. Y. 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 front of the latter there seems to be a thin, narrow 
bone (see Xo. 19804a, 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 (PI. Y. 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, Dut not by any ginglvmoid 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 
