144 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



are, however, indications of four, and perhaps five, pairs of short, thick, cephalic appendages on the 

 ajiterior tlurd 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 hindenuost ((5th) 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 

 ari", lead us to suppose that they approached iu 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 caual ; it is swollen in front, the dilatation probably i-epresentine: the pro- 

 ventriculus, and in outline the cast recalls that of the digestive caual of Limulus. Judging by 

 analogy, the mouth was probably, as in the larval Limulus, situated well iu front between fhe 

 anterior pairs of appendages, and the (jesophagus 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. 



.Judgingby our specimens and Dr. Woodward's figures, Cyclus if restored would have au 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 thiu 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 eventu.ally 

 becomes the caudal spine. The Cyclus was provided with sis pairs of cejihalic appendages, which 

 were short, not reaching beyond the edge of the body. With these the animal could creep over the 

 bottom of the shallow, muddj' ])ortions 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 embryo 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 17« or IS, 1S«, and perhaps 19 and 19rt, of our memoir* of one of which 

 (19a) Fig. 8 is a copy. At this stage of development the body of Limulus is 

 hemispherical ; seen from beneath the ontline of the body is nearly orbicular, the 

 abdominal region completing the circle. If Limulus were arrested at the stage of 

 development when only three abdominal segments had appeared, and the devel- 

 opment of the teet and claws had been accelerated and then hatched, it would be, 

 so to speak, a Cyclus. 



^Li'inuiii^^"c y c° u°s ' ^" our first memoir on the development of Limulus we adopted Dr. Wood- 

 '**'''5e- ward's view tliat Cyclus was a Xiphosuran. In ISGS Dr. Woodward stated : "We 



must differ from M. de Konink iu referring this form to the Trilobites. If truly au 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 contractUe, its original segments 

 being completely soldered together into one piece " ; and in 1870 he adds that, from the recent inves- 



' The Development of Limulus polypheniiis, 1H72, PI. iv. Memoirs Bast. Soc. Nat. Hist., Vol. 1. 



Since this article was seut to the printer, I have received, through the kindness of the author, Jlr. B. N. 

 Peach's " Further Researches among the Crustacea and Arachuida of the Caiboniferous Rocks of the Scottish Border. 

 Trans. Roy. Soc. Edinburgh, 1882." In this memoir Mr. Peach figures aud briefly describes the limbs of Cyclns. 

 " From 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 coxie." Under Cijdua testudo Peach, he describes six triangular plates on 

 each side, divided from each otlier by deep sulci, and converging upou an oral sternum. He also refers to " the jointed 

 cylindrical limbs, the tips of which have not been observed." 



