ON THE STRUCTURE AND CLASSIFICATION OF THE FOSSIL CRUSTACEA. 70 
Cyclus radialis (Pl. I. fig. 1) is an elegant little shield-shaped buckler 
5 lines long by 4 in breadth; its general form is hemispherical, with a 
narrow smooth border; the shield is divided down its centre by a raised 
longitudinal ridge, from which radiate seven diverging ribs whose rounded 
ends reach the lateral and posterior border. 
The anterior cephalic portion occupies about a quarter of the entire shield, 
and is ornamented by the spreading out of the raised central ridge, and by 
two subcentral rounded prominences which correspond in position to eye- 
spots, but are not facetted. The ribs are ornamented each with from three 
to five tubercles irregularly disposed over their surface. 
The new form of Cyclus (Pl. II. fig. 2) discovered by Dr. Rankine of 
Carluke, in the Carboniferous shales of that place, is most remarkable in 
appearance, and certainly far more like a parasitical Crustacean than the 
Cyclus radialis, which certainly seems to have been furnished with a hard 
calcareous test. A comparison of the two, however, leaves no doubt in my 
mind in referring them both to one genus. 
The shield is about 4 lines in diameter, and conveys the idea of an ex- 
‘tremely thin test flattened out on the soft shale by pressure. The eye-spots 
occupy the same relative position as in C. radialis ; but the divisions which 
represent the costs are six, not seven in number in this species, and these 
anastomose together on the lateral border, and diverge, not from a median 
raised ridge, but a broad V-shaped central area. One is reminded by this 
Crustacean of the appearance of Argulus, Bopyrus, and other recent parasitic 
forms, and also of the disk-shaped Discinocaris, from which it differs, how- 
ever, in the prominent eyes and costated shield. 
For this new species (Plate II. fig. 2) I propose the name of Cyclus Ran- 
Kini, after its discoverer. 
In describing Cyclus radialis, M. de Koninck observes :— 
“There is no doubt this animal should be ranged with the Crustacea, 
and in Milne-Edwards’s order Trilobita abnormalia and battoidea, near to 
Agnostus.” , 
M. de Koninck also thinks it probable that the body of Cyclus was soft and 
very contractile, that it was a parasite, and that the two tubercles which 
we have called the eyes really covered those organs—and, further, that the 
ribbed border protected the feet when the animal was in repose. 
We must differ from M. de Koninck in referring this form to the Trilobita. 
If truly an adult, it must be placed near to Apus with the other shield- 
bearing Phyllopoda ; if a larval form, it may have been the early stage of 
Prestwichia or some other of the Coal-measures Limulide. Nor do we think 
‘it in the least probable that the shield of Cyclus radialis was flexible or con- 
tractile, its original segments being completely soldered together into one 
piece. 
Hermann von Meyer has figured a small Crustacean head-shield under the 
name of Halicyne agnota, and a second species, H. lawa*, from the Mus- 
chelkalk of Rottweil in Germany.. Goldfuss originally figured it as an 
Olenus (O. serotinus); afterwards it was referred to Limulus by Miinster 
_ (Beitriige, 1841, Bd. i.t. v. f.1). To both these conclusions Meyer demurs— 
to Limulus because no eyes are visible, and to the Trilobita because none are 
found older than the Carboniferous. 
The form of this head-shield is extremely like that of Agnostus; but the 
Aynostide are confined to the Lower Silurian strata, between which and the 
* See Palaontographica, 1847, vol. i. p. 134. 
1868. G 
