4 G. LINDSTRÖM, A SILURIAN CYATHASPIS FROM GOTLAND. 
dise and, moreover, through faint traces of partition lines. 
In a vertical section, across these lines, they are not discer- 
nible below the surface, but have interiorly coalesced with the 
central disc, so as to form a single, solid piece (P1. II, fig. 
2 & d KÖ : 
The rostrum is transverse, with the anterior margin 
arehed, the posterior one forming a curved or wavy line. 
The large central disc is anteriorly narrow and widens 
towards the posterior margin, where it is protracted into a 
short and blunt central spine. 
The lateral plates (cornua) are narrow, broader ante- 
riorly, tapering off backwards and ending in a small sharp 
point. Their edges are inflected towards the interior surface 
and in consequence the glossy ridges continue a little below 
on the inferior surface. When seen from the sides they show 
near the anterior margin a shallow notch, which may have 
formed the upper part of the orbit (Pl. I, fig. 4). 
The ridges of the surface are generally parallel to the 
longitudinal axis of the plates which they cover. 'Thus in 
the rostral plate they are tramsverse or go in a direction at 
right angles to the ridges of the hindermost plates. Seldom, 
excepting on the narrow cornua, can a ridge be seen conti- 
nuing without interruption the whole length of the plate. 
They cease or are intertwined with other or are dissolved, 
as it were, in groups of small tubercles or knobs. As several 
authors have most appropriately said, their general appea- 
rance reminds one of the markings of the epidermis on the 
palm of the human hand. On the large central disc is re- 
marked a very distinct triangular area, having its base to- 
wards the anterior margin and its apex backwards. The ridges 
there form irregular whorls and are dissolved into a small 
group of tubercles, like a sort of pavement. 
The interior surface, or that which must have rested 
immediately upon the soft organic tissues, (P1. I, fig. 12) by 
which it has been secreted, is, as far as may be seen from the 
detached fragments, smooth, of a sombre colour, and is covered 
with innumerable microscopical openings of narrow tubes or 
ducts, which perforate the shell substance. It is moreover 
divided into polygons by minute lines, no doubt corresponding 
to the partition walls of the vacuoles mentioned below and 
which form the chief mass of the shield. 
