150 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



SYNOPSIS OF THE NORTH AMERICAN XIPHOSURA. 



Suborder XIPHOSURA. 



Family 1. Cyclid^ Pack. 



Body disk-like, orbicular; abdomen composed of three segments, scarcely if at all diftereii- 

 tiated fi-om the cephalic shield; cephalic limbs nearly as in the larval Limulus; size small. 

 Genus Cyclus De Koninck, with the characters of the family. 



Gyclus americanus Pack. 

 Family 2. Dipeltid^ Pack. 



Body disk-like, elliptical ; abdomen differentiated from the cephalic shield, smooth, no seg 

 ments indicated. 



Genus Bipeltis Packard, with the characters of the family. 



Dipeltis diplodiscus Pack. 



Family 3. Belinurid^ Pack. 



Body limuloid in general shape; cephalic limbs as in the larval Limulus; shield with long 

 slender genal spines ; abdomen with the segments distinct; caudal spine short or long. 



Genus Frestwichia Woodward. Eight abdominal segments, including the short caudal spine. 



Prestwichia dance Meek. 

 Prestwichia longispina Pack. 

 Genus Belinurus Konig. Nine abdominal segments, including the very long, slender caudal 



spine ; segments 7 and 8 consolidated. 



Belinurus lacoe'i Pack. 



Family 4. Limulid^ Zittel. 



Body longer than broad ; abdomen with segments consolidated ; six pairs of abdominal limbs, 

 Ave pairs having over a hundred jjairs of gill-leaves. 



Genus ProtoUmulus Packard.* Seven abdominal segments, including the large thick caudal 

 spine. 



ProtoUmulus eriensis (Williams). 

 Genus Limulus Miiller. Cephalic limbs large ; body longer than broad ; abdomen with 9 seg- 

 ments ; caudal spine longer than the body. 



Limulus polyphemus {Linu.).t 



* In a notice of a new Limuloid Crustacean from the Devonian, Amer. Journ. Sc, July, 1885, p. 45, Prof. H. S. 

 Williams described an interesting Limuloid from the Devouian of Erie Couuty, Peunsylvania (associated with typioal 

 Chemunglbssils). Itis described as Prestwichia eriensis, the author remarking that "its identitication with Prestwichia 

 must be regarded as provisional." He then adds : " The following characters exhibited iu the sijecimeu are regarded as 

 generic and as locating it with genus Prestwichia of Woodward : (1) the elliptical head shield ; (2) the genal spines which 

 proceed backwards more directly than in any described species of the genns ; (3) the thoracico-.abdominal segments auchy- 

 losed to form a buckler, to which is attached (4) a longtelson. The general outline of the whole animal resembles that 

 of the modern Limulus." We have ventured, without having seen the specimen, to regard this form as probably a mem- 

 ber of the family Limulidic, and the forerunuer of Limulus. It is certainly not a Prestwichia. The body is apparently 

 longer than broad, and in outline it leaves a strong resemblance to the young Limulus after its first moult. This is 

 seen in the shape of the .abdomen and the caudal spine and in their relations to the rest of the body. It also seems 

 probable that the abdominal segments were not free; in this respect it diflers from the Belinuridaj, especially Prest- 

 wichia. Judging by the number of lateral spines, the abdomen was composed of 11 segments exclusive of the caudal 

 spine, thus differing from Prestwichia, which has 7, also from Limulus, which has8 pairs of lateral spines. We therefore 

 venture to give it the generic name of ProtoUmulus, and to regard it as standing at the base of the family to whicli 

 Limulus belongs. Its occurrence in the Devonian makes it a connecting link between the Upper Silurian Neolimulus 

 and the Carboniferous and Jurassic Limuloids. We are indebted to Prof. Williams for the use of figures illustrating 

 his P. eriensis. 



t Besides the American species, there are three others living, viz, L. moluecanun ; (East India) L. longispina Van 

 der Hoven, Japan ; L. rotundicauda Latr., Molucca Is. and Malacca. 



