THE GENUS HALICYNE. 255 



larval form of Crustacean than is the Cyclus radialis, which certainly possessed a hard 

 calcareous test. A comparison of the two, however, leaves no doubt in my mind in 

 referring them both to the same genus. 



The shield is about 4 lines in diameter, and conveys the idea of an extremely 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 costse are six, not seven, 

 in number in this species, and these anastomose together on the lateral border, and diverge, 

 not from a raised median ridge, but from a broad, V-shaped central area (cardiac region?). 

 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, however, it 

 differs in the " eye-spots " and costated shield. 



For this new species I propose the name of Cyclus Rankini, after its discoverer. 



[Halicyne, von Meyer, 1847. 



Hermann von Meyer has figured in the ' Palseontographica,' 1847, vol. i, p. 134, a 

 small Crustacean head-shield under the name of Halicyne aynota, and a second species 

 under the name H. lawa, from the Muschelkalk of Rottweill in Germany. Goldfuss 

 originally figured it as Olenus serotinus; afterwards Minister referred it to Limulus 

 ('Beitrage,' 1841, Bd. i, t. v, f. 1). 



To both these conclusions von Meyer demurs ; to Limulus because no eyes are visible, 

 and to the Trilobita because none are found in rocks younger than the Carboniferous 

 formation. 



The form of this head-shield is extremely like that of Agnostus ; but the Agnostida 

 are confined to the Lower Silurian strata, between which and the Trias are the long 

 intervening series of the Upper Silurian, the Devonian, Carboniferous, and Permian 

 formations. At the date of my report (1868) I was inclined to place Halicyne with 

 Bunodes, Hemiaspis, &c, among the aberrant forms of the Limulidce, being under the 

 impression that it was only a part of a Crustacean, probably a head-shield. Having now 

 the advantage of a large series of Carboniferous Cycli with which to compare it, I feel no 

 longer any doubt about placing it with Cyclus, that being the older name.] 



