Description of Two Species of
Cephalaspis
Authors: Harley John 
Published: October 2009

Abstract

A new form of
Cephalaspis
(C.
Asterolepis
) was found by the author about two years ago at Hopton Gate, about four miles east of Ludlow, in a bed of coarse Old Red conglomerate overlying a compact calcareous sandstone, which latter abounds with fragments of
Cephalaspis Lyellii
and
Pteraspis
, and in its upper part contains remains of
Pterygotus problematicus
? The former is horizontally bedded, and alternates with a brown micaceous shale, in which are carbonized fragments of plants.

Cephalaspis Asterolepis
from the Old Red Sandstone of the neighbourhood of Ludlow is by far the largest species that has been discovered, the cephalic plate having at least twice the dimensions of that of
C. Lyellii
. In outline it is broadly semielliptical. In addition to its large size, it is distinguished by the position, obliquity, and magnitude of the orbits. Placed almost entirely on the posterior half of the shield, these diverge from each other so as to be distant 1 inch apart anteriorly, and ½ an inch posteriorly. The long diameter of each orbit measures 1 ¼ inch. Within the orbital circumference remains of the osseous sclerotica are visible. Owing to the backward position of the orbits, it is in the posterior of the shield that we find some of the chief distinctive modifications: thsu the occipital crest is even shorter than that of a species less than half its size; the space between the orbital ridges is proportonately small.

The outer enamel-layer is ornamented with tubercles, which bear so close a resemblance to those covering the bony plates of
Asterolepis
as to have suggested the specific name. They present, however, considerable variation: usually they arise by a circular and more or less tumid base, which gradually terminates in an elevated papilla; or the papilla may be small, rounded, and distinct from the base. Sometimes two or even three minute papillæ surmount the same base; sometimes the base is quite flat and expanded, and is occupied by a solitary central papilla. In all cases the base presents more or less distinctly a radiated striation. The inner layer of the bony plate presents lacunæ and long branching canaliculi precisely resembling those of human bone. Many of these are completely injected with a transparent blood-red material; and so beautifully are they displayed, that one ignorant of the structure of bone would be able to apprehend it by a glane at a minute part of this ancient fragment.

So wonderfully indeed has nature treasured up her secrets in this disentombed relic of a time so distant as to be incalculable, that she distinctly reveals in their minutest details the structure of canals not more than the
one-fifty-thousanth of an inch in diameter, and such as defy the skill of the anatomist to inject.

Mr. Harley also described a more perfect specimen of
Cephalaspis Salweyi
than
the one on which Sir P. Egerton not long since determined the species*. It was found by Mr.
Salwey at Hinstone near Bromyard, and about 1 ½ mile from Acton Beauchamp, in a sandtone similar to that in which the other specimen was found at Acton Beauchamp.

Cephalaspis Salweyi
proves to differ but very little in size and external configuration from
C. Lyellii
. It is distinguished from this species by its short and slender postero-lateral spines, which are not prolonged further backwards than the termination of the occipital crest. The internal parts of the posterior third of the shield form a distinct, arched, nuchal plate, which is prolonged posteriorly. The occipital crest is prominent, and bifurcates anteriorly into the orbital ridges. These, diverging, curve a little outwards and pass to the posterior margins of the orbits, enclosing between them a tabulated surface, which, occupying the central and most elevated parts of the general surface, forms a prominent feature. The orbits are small and slightly oval; they are situated entirely on the
anterior
half of the shield. The enamel-layer forms small, oblong or round, pearly drop-like tubercles, which are numerous an for the most part discrete. The space between them are grooved into parallel or slightly divergent ridges, which pass from their sides and ends. The structure of these tubercles is very vascular.

Associated with this fossil, the author found the dermal plate or the tooth of a placoid fish, which has close resemblance to the teeth of
Cestracion
and the Rays, and even closer still with the bone of the Silurian bodies called by Pander
Cœlolepidœ
. It was discovered almost between the jaws of
C. Salweyi
, and thus suggests the paradoxical question—does it belong to that fish?