94 WHITEAVES ON FOSSIL FISHES FEOM 



posteriorly, slightly concave anteriorly, and with a small, but distinct notch (b) a little 

 behind the middle. When the rostral plate (c) is absent, -which is almost invariably the 

 case, the anterior margin is concavely emarginate in the centre, the emarginatiou being 

 broad, transverse and bounded on each side by an obtusely triangular projection (d). 

 On the outer side of each of these projections there is an obliquely and shallowly con- 

 cave, lateral emargination. In one specimen only (that from which the woodcut and 

 fig. 1 on Plate IX were made) the rostral plate (t) fits into and completely fills up the 

 central emargination of the front margin of the shield. This plate, which is nearly 

 twice as broad as long, projects beyond the front margin of the shield, its two sides are 

 narrowly rounded, but its anterior margin is broken. Test very thin. Sculpture con- 

 sisting of numerous small conical tubercles which are smooth at their summits and 

 marked with fine radiating grooves below. On some of the bony plates of the shield 

 the tubercles are isolated and scattered, but in others they are arranged very distinctly 

 in concentric lines separated by continuous furrows. Besides the tubercles, the surface 

 is marked by certain superficial grooves, which are represented in the woodcut by 

 unbroken lines. The general direction of most of these grooves is longitudinal, and 

 the most strongly marked are those which run from the autero-lateral (e.e.) to the 

 postero-lateral angles (a.a.) and which are nearly parallel to the sides of the shield. 

 Sutures scarcely perceptible, their supposed outlines being indicated in the figure by 

 dotted lines. 



Posl-dorsomedian plate (Fig. 2.) — Convex along the median line but highest in the 

 centre, from which point there is a downward slope in every direction, the lateral slopes 

 being most abrupt. Outline oblong but narrowing posteriorly so as to form a short beak. 

 Anterior end somewhat rounded ; sides parallel for more than two-thirds of their length, 

 then attenuating rapidly into a point with obliquely concave sides. Maximum breadth 

 equal to about one-third the entire length ; apex of the beaked extremity curved slight- 

 ly upwards. Tubercles arranged concentrically but not in distinct rows, those in the 

 centre being the smallest and those near the circumference being both distant and of 

 comparatively large size. 



Preventrulateral plates (Fig. 3.) — Flat ; longitudinally subreniform, a little longer than 

 broad ; outer margin concavely emarginate and inflected. Tubercles isolated, crowded 

 and arranged obscurely in concentric, subparallel lines. 



Veniromedian plate (Fig. 4.) — Flat ; subrhomboidal, but with all the sides ixn equal and 

 the margins of two of them (the right anteriorly and the left posteriorly) shallowly con- 

 concave. Posterior extremity rather more produced than the anterior ; length about one- 

 third greater than the breadth. Tubercles arranged in distinct rows on three sides, but 

 not on the left side of the posterior half, where they are nearly all isolated, those to- 

 wards the centre being comparatively large, and those near the centre very minute and 

 densely crowded. 



More than twenty well preserved and tolerably perfect specimens of the cranial 

 shield have been collected, besides numerous fragments, but the suborbital plate is inva- 

 riably absent and the supposed rostral plate is only preserved in place in one specimen. 

 The whole of these shields, too, appear to have been flattened by pressure, and if so, 

 they may once have been longer in proportion to their breadth than they are now, and 

 the anterior sinus into which the rostral plate fits may have been narrower and deeper. 



