GEOLOGICAL SOCIETY OF DUBLIN. 
169 
brookdale is easily recognised, and the specimens, so far, are sufficiently 
well preserved for description. 
Before doing so, however, I would offer a few remarks on the genus 
Limulus, in which all these coal-measure Crustacea have been hitherto 
included, and which I now propose, from their greater affinity with the 
Trilobites, to remove, and constitute a new genus under the name of 
Steropis, for the following reasons. In the first place, their general form 
and size bear a much, stronger resemblance to several of the Trilobites 
than they do to the recent Limulus, from which it differs in possessing 
(although not so perfect as in the Trilobites) a more distinct trilobation, 
with the abdomen separated into segments. The abdominal or caudal 
shield corresponds almost completely in point of size and form with 
that of Amphyx, Trinucleus, &c.; and the characteristic spiny termina¬ 
tion of the pleurse to that of Acidaspis and Paradoxides. The possession 
of legs and articulation of the caudal spine, which they are said to be 
provided with, would connect them with the Jurassic and recent Li¬ 
mulus ; although there is a striking analogy to the latter case presented 
by some of the Silurian Trilobites, as Phacops longicaudata, in which 
species there is a great prolongation of the caudal extremity into a spine, 
which is, however, destitute of articulation. 
The presence of a facial suture, which I have detected in the species 
hereafter described, would offer still greater affinity to the Trilobites, as 
being peculiarly characteristic of that group. 
The great difference, in point of time, between the deposit of the 
lower coal-measures, in which Crustacea of this character first appears, 
and the upper Jurassic, where they approach very closely to the recent 
forms, would again account for their closer alliance to the Trilobites, 
thus leading on in beautiful gradation to that great and important group 
of the Crustacea which is characteristic, and obtains their maximum de¬ 
velopment in the older palaeozoic rocks. 
Two species of the genus Limulus are recorded as occurring in the 
Muschelkalk, but in the upper Jurassic formation they are found in fine 
preservation, six species having been described from the cream-coloured 
slates of Solenhofen and Pappenheim. They approach much nearer in size 
and form to the recent Limulus, having distinct legs, with an evidently 
articulated tail,—the differences, therefore, being so slight, it would, per¬ 
haps, be advisable to retain them in the same genus. 
The recent forms of Limulus, to which the King Crab belongs, interest¬ 
ing from its relation to these ancient Crustacea, are now most abundant 
in the seas of warm climates, chiefly in those of India, and on the coasts 
of America. 
The following is the description of the species from the coal-measures, 
Bilboa Colliery, county of Carlow. 
Steropis arcualus, n. s. 
Cephalic shield, semicircular or lunate, slightly arched, declining 
towards the circumference, and surrounded by a narrow margin which 
is destitute of spines at its anterior extremity; the central portion, head or 
