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 (operculum) which covers 

 them. In Limulus there are no salivary glands, no MalpigTiian 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. 1 



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 are 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- 

 careous or chitinous, bounded by a distinct border. Appendages 

 nearly as in embryonic Limulus. 



FlG 1411 Cyclus, de Kon. (Fig. 1411). Known almost solely by 



Cydus americanus, Pack. tne cephalothorax with its imperfectly preserved appendages, 



Coal Measures ; Mazon Creek, which seem to be simple swimming legs. Their enlarged 



Illinois. Showing traces of . . , . 



legs and alimentary canal. Vi- J in ts 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, etc. 



1 Literature: 



Hoeven, J. van der, Recherclies sur 1'histoire naturelle et 1'anatomie des Limules. Leyden, 

 1838. Milnster, G. Graf zu, Beitrage 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. Giebel, C. G., Limulus Decheni (Zeitschr. gesammt. Naturw. 

 XXL), 1863. Meek, F. B., and Worthen, A. H., Report Geol. Surv. Illinois, vol. III. p. 547, 

 1868. Woodivard, 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.), ISTZ.Dohrn, A., Embryo- 

 logie und Morphologic 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 (Anuiv. Mem. Boston Soc. Nat. Hist.), 1880. Carboniferous Xiphosurous Fauna of North 

 America (Mem. Nat. Acad. Sci., III.), 1885. Milne- Edwards, A., Recherches sur 1'anatomie des 

 Limulus (Ann. Sci. Natur, Zool. [5], XVII.), 1873. Lankester, E. R., Limulus an Arachnid (Quar. 

 Journ. Microsc. Soc., XXI.), 1881. Peach, B. JV., 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. [3], 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. Kingsley, J. S., Embryology of Limulus (Journ. Morphol., VII., VIII.), 1892-93. 



