670 ARTHROPODA SUB-KINGDOM VII 
joints for prehension and mastication. Behind the mouth is a single or paired metastoma, 
Cephalothorax depressed, with usually a pair each of median ocelli and lateral compound 
eyes. Respiration by means of lamellate branchiae (“ gill-books”) borne on appendages two 
to six of the abdomen, and protected by the enlarged first pair (operculwm) which covers 
them. In Limulus there are no salivary glands, no Malpighian tubules, and no embryonic 
membranes (“amnion”) are found in development. 
The prevailing modern opinion is that the body is divided into two regions only 
(cephalothorax and abdomen) instead of into three (head-shield, “thorax,” and 
abdomen), as a number of earlier writers have maintained. According to the latter 
interpretation, the abdomen of Limulus is represented merely by the telson, but in 
Eurypterids by a variable number of segments. 
Order 1. XIPHOSURA. Gronovius.! 
Body, in mature types, distinctly trilobed longitudinally. Cephalothorax large, semi- 
circular, the compound eyes laterally situated, and ocelli in the centre in front. First pair 
of appendages chelicerate. Metastoma with two small accessory plates. Abdomen with seven 
to ten segments, which wre dorsally free or coalesced; the six 
anterior ones provided with lamellar appendages on the under 
side. Telson long, ensiform, movable. 
Family 1. Cyclidae. Packard. 
Cephalothorax small, orbicular, discoidal or convex, cal- 
carcous or chitinous, bounded by a distinct border. Appendages 
nearly as tn embryonic Limulus. 
Cyclus, de Kon. (Fig. 1411). Known almost solely by 
Pack, the cephalothorax with its imperfectly preserved appendages, 

Fic. 1411. 
Cyclus americanus, 
Coal Measures; Mazon Creek, which seem to be simple swimming legs. Their enlarged 
Illinois. Showing traces of 
j,. joints cover the ventral surface of the carapace everywhere 
except in the centre, which is occupied by a V-shaped plate, 
towards the pointed extremity of which all the basal joints of the limbs converge. 
Coal Measures; Great Britain and Illinois. 
Some of the described species are apparently larval Eurypterus, Stylonurus, 
Prestwichia, ete. 
legs and alimentary canal. 1 
1 Literature : 
Hoeven, J. van der, Recherches sur Vhistoire naturelle et l’anatomie des Limules. Leyden, 
1838.—Miinster, G. Graf zu, Beitrige zur Petrefaktenkunde, Hefte I., III., 1840.—Gegenbaur, C., 
Anatomische Untersuchungen eines Limulus (Abhandl. naturf. Ges. Halle, IV.), 1858.—Baily, W. H., 
Explanation of Sheet 137 of the Maps of the Geol. Surv. Scotland, 1859.—Remarks on Belinurus 
(Ann. Mag. Nat. Hist. [3], XI.), 1863.—Giebe/, C. G., Limulus Decheni (Zeitschr. gesammt. Naturw. 
XXI.), 1863.—Meek, F. B., and Worthen, A. H., Report Geol. Surv, Illinois, vol. IIL. p. 547, 
1868.— Woodward, H., On Neolimulus (Geol. Mag., V.), 1868.—On the Genus Cyclus (ibid. VII. p. 
554; also [4], I. p. 530), 1870-94.—Notes on Merostomata (ibid. IX.), 1872.—Dohrn, A., Embryo- 
logie und Morphologie des Limulus (Jenaische Zeitschr., VI.), 1871.—Packard, A. S., Development 
of Limulus (Mem. Boston Soc. Nat. Hist., I.), 1872.—Anatomy, Histology, and Embryology of 
Limulus (Anniv. Mem. Boston Soc. Nat. Hist.), 1880,—Carboniferous Xiphosurous Fauna of North 
America (Mem. Nat. Acad. Sci., IIT.), 1885.—Milne-Edwards, A., Recherches sur l’anatomie des 
Limulus (Ann. Sci, Natur. Zool. [5], XVIL.), 1878.—ZLankester, L. R., Limulus an Arachnid (Quar. 
Journ. Microse. Soc., XXI.), 1881.—Peach, B. V., Further Researches among Crustacea and Arach- 
nida (Trans. Roy. Soc., Edinburgh, XXX.), 1882.— Williams, H. S., New Limuloid Crustacean from 
the Devonian (Amer. Journ. Sci. [8], XXX.), 1885.—Hall, J., and Clarke, J. M., Palaeontology of 
New York, VII. p. 153, 1888.—Kishenouye, K., Development of Limulus (Journ. Coll. Sci. Tokyo, 
V.), 1891.—Aingsley, J. S., Embryology of Limulus (Journ. Morphol., VII., VIII), 1892-93. 
