144 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
are, however, indications of four, and perhaps five, pairs of short, thick, cephalic appendages on the 
anterior third of the body. Unfortunately, they are not well preserved, the basal and distal por- 
tions not present, and the indications of joints indistinct; they are directed outwards from near the 
median line of the body, on each side of the intestine, the hindermost (6th) pair being directed 
somewhat obliquely outwards and backwards. In their position and relative distance apart they 
seem homologous with the cephalic limbs of the larval Limulus. The indications, slight as they 
are, lead us to suppose that they approached in general shape and relative size those of Prest- 
wichia, reaching near but not passing beyond the edge of the cephalic shield. The distal portion of 
the limbs not being preserved, it is impossible to conjecture whether they were forcipicate or not.* 
Through the middle of the body, from near the anterior to the posterior margin, passes the 
cast of the digestive canal; it is swollen in front, the dilatation probably representing the pro- 
ventriculus, and in outline the cast recalls that of the digestive canal of Limulus. Judging by 
analogy, the mouth was probably, as in the larval Limulus, situated well in front between the 
anterior pairs of appendages, and the csophagus curved forward and upward from the mouth, 
while the vent was situated very near the hinder edge of the body. 
There are no distinct traces of an abdominal region in the specimen, and it will be seen that 
in some of Dr. Woodward’s figures there is also none. It is not probable that there was any spine 
in the genus, none being indicated in any of the figures or descriptions published. 
Length of body, 14°; breadth, 14"; width of the flattened rim or margin, 1". Locality, 
Mazon Creek. No. 218a, b. Collection of Mr. Lacoe. 
Judging by our specimens and Dr. Woodward’s figures, Cyelus if restored would have an orbic- 
ular body, more or less disk-like or hemispherical, with a cephalic region composed of six seg- 
ments, which are not, however, indicated externally; this region had a thin margin, as in Prest- 
wichia and Limulus, <A pair of median ocelli were probably present, but no compound lateral eyes 
have yet been discovered. An abdominal region was slightly differentiated, and it was composed 
of three segments, the third representing that of the embryo Limulus, which in that form eventually 
becomes the caudal spine. The Cyclus was provided with six pairs of cephalic appendages, which 
were short, not reaching beyond the edge of the body. With these the animal could creep over the 
bottom of the shallow, muddy portions of the carboniferous sea. It is not improbable that there 
were two pairs of abdominal lamellated legs, adapted for respiration, short and broad, and not 
unlike those of the empryo Limulus. In fact, our conception of the form of the living Cyclus is 
that it was not much unlike the advanced embryo of Limulus, either in the stage represented in 
Figs. 17 and 17a or 18, 18a, and perhaps 19 and 19a, of our memoir* of one of which 
(19a) Fig. 8 is a copy. At this stage of development the body of Limulus is 
“72 \ hemispherical ; Seen from beneath the outline of the body is nearly orbicular, the 
SS CY) abdominal region completing the cirele. If Limulus were arrested at the stage of 
SS development when only three abdominal segments had appeared, and the devel- 
SS? opment of the feet and claws had beén accelerated and then hatched, it would be, 
so to speak, a Cyclus. 
We ice ie In our first memoir on the development of Limulus we adopted Dr. Wood- 
Brae eo: ward’s view that Cyclus was a Xiphosuran. In 1868 Dr. Woodward stated: “ We 
must differ from M. de Konink in referring tiis form to the Prilobites. If truly an adult, it must be 
placed near to Apus, with the other shield-bearing Phyllopods; if a larval form, it may have been 
the early stage of Prestwichia, or some other Limuloid of the coal-measures. Nor do we think it in 
the least probable that the shield of Cyclus radialis was flexible or contractile, its original segments 
being completely soldered together into one pire ”; and in 1870 he adds that, from the recent inves- 
* The ee Siopment of Teimaiae apn GAS. 1872, Pl iv. momen Bost. Soe. _ Nat. Hist., Vol. We 
Since this article was sent to the printer, I have reeeiyed, through the kindness of ime author, Mr. B. N. 
Peach’s ‘‘ Further Researches among the Crustacea and Arachnida of the Carboniferous Rocks of the Scottish Border. 
Trans. Roy. Soe. Edinburgh, 1882.” In this memoir Mr. Peach figures and briefly describes the limbs of Cyclus. 
‘Prom the fact,” he says, “ that several of the Survey specimens exhibit limbs, the radiating lines of the sternum are 
most probably the divisions between the cox:e.” Under Cyclus testudo Peach, he describes six triangular plates on 
each side, divided from each other by deep sulci, and converging upon an oral sternum. He also refers to ‘‘ the jointed 
cylindrical limbs, the tips of which have not been observed.” 
