672 ARTHROPODA SUB-KINGDOM VII 
Family 3. Limulidae. Zittel. (King or Horseshoe Crabs.) 
Body longer than broad ; cephalothoraz arched dorsally, the central portion separated 
from the sides by longitudinal grooves; marginal area large and flat. Abdomen composed 
of six consolidated segments forming a simple sub-triangular shield, and a long slender 
telson. Six pairs of abdominal limbs, five of them having over a hundred pairs of gill- 
leaves. 
Limulus, Miller (Fig. 1413). Living species belonging to this, the only genus of 
the family, occur on the eastern shores of America and Asia. The four cephalothoracic 
feet are chelate, the sixth pair furnished with a whorl of plates used in pushing the 
animal through the mud. Gills are borne upon the five posterior pairs of abdominal 
appendages, the anterior pair being without gills, but having the genital opening upon 
the posterior face. A small species of Limulus occurs in the Buntersandstein of the 
Vosges. L. Walchi is abundant in the Lithographic Slates of Bavaria; L. Decheni is 
found in the Oligocene brown coal of Teuchern, near Merseburg. 
Order 2. SYNXIPHOSURA. Packard. 
Body elongated ; cephalothoraz semicircular with the median portion more or less 
distinctly defined. Compound eyes present in some forms, ocelli not observed except in 
Neolimulus. Abdomen distinctly trilobed, tts segments free, the 
pleurae flat and extended, and (except in Bunodes) terminating in 
lateral projections or spines. 
With the exception of the Cambrian Aglaspis, all the genera be- 
longing to this order are of Silurian age, and are too imperfectly 
known as yet to permit a satisfactory grouping into families, although 
several such have been proposed by Packard. Zittel united them, 
together with certain genera of Xiphosura, in the family Hemiaspidae, 
and this term is retained with the exclusion of the Xiphosurous forms 
and Aglaspis. The latter is without facial sutures. 

Family 1. Aglaspidae. Clarke. 
Fic. 1414. 
Aglaspts Hatont, Whitt Cephalothorax moderately large, trilobed ; abdominal segments 
Upper Cambrian; Lodi, with distinct axis and plewrae ; telson long, spiniform. 
Wisconsin. %/}. 
Aglaspis, Hall (Fig. 1414). Cephalothorax with central 
portion short and conate, in front of which are approximate com- 
pound eyes ; bounded on all sides by a distinct border. Abdominal 
segments not grooved on the pleurae. Cambrian ; Wisconsin. 
Family 2. Hemiaspidae. Zittel. 
A somewhat heterogeneous group, separated by at least a 
family difference from the Aglaspidae. 
Neolimulus, Woodw. Cephalothorax short and broad, crescentic, 
elevated mesially, and bearing ocelli. Compound eyes lateral, and 
connected with the genal angles by a suture. Abdomen very 
broad, composed of at least nine trilobed segments ; telson not 
observed. This genus connects the Avphosura with the Syna- 
. . fer Bunodes tunula, var. 
phosura. Silurian ; Scotland. Schrenii, Nieszk. ‘Silat 
Bunodes, Eichw. (Exapinurus, Nieszk.), (Fig. 1415). Cephalo- ion 5 00 ee 
thorax semicircular, convex, with radial furrows from the median stored from: » aneHi 
portion. Eyes wanting, and no genal spines. Facial sutures sect, (after F. 
obscure, converging from the posterior to the anterior margin. 
Abdomen divided into an anterior portion (“thorax”) consisting of six trilobite-like 

Fig, 1415. 
