MUSEUM OF COMPARATH'E ZOOLOGY. 205 



The mode of attachment of the leg to the ventral surface is shown in 

 the transverse section for Cabjmene in Plate III. fig. 9, and in Ctravr- 

 rus in Plate II. fig. 3. The longitudinal section is given in Plate V. 

 figs. 1 and 3, for Calyniene, and in Plate II. fig. 6, for Ceraurus. 

 These illustrations are considered as showing that the point of articula- 

 tion was a small, round process projecting from the posterior surface of 

 the large basal joint, and articulating in the ventral arch somewhat as 

 the legs of some of the Isopods articulate with the arches in the ventral 

 membrane. The arches of the ventral membrane in the Trilobite, and 

 the parts shown in Plate IV. fig. 7, afford a correspondingly firm 

 basis for the attachment of the legs. The general curvature of the 

 legs is forward, as shown by all the sections, when they are attached 

 to the ventral surface and in their normal position. This corresponds 

 with the position of the cephalic appendages, and gives a uniformity 

 to the entire series. 



Our knowledge of the number of pairs of appendages is based on the 

 evidence given by sections of Calymene, Plate V. figs. 1-4. The 

 dorsal shell of this species has thirteen segments in the thorax, and 

 nine coalesced in the pygidium. The section of the median lobe and 

 ventral surface. Fig. 2, shows twenty arches, and Figs. 1 and 2, sections 

 of the same individual, show twenty and twenty-two thoracico-abdominal 

 appendages respectively. The fact that there is a space between the 

 last arch or appendage and the posterior margin of the p^'gidium does 

 not necessarily prove the existence of other appendages, as it is quite 

 probable that in the process of disintegration of the visceral cavity the 

 entire ventral membrane, with its attached legs, was drawn away from 

 the pygidium by the pressure of the sediment imbedding it. This 

 view is strengthened by Fig. 4, Plate V., as there are but eighteen or 

 nineteen appendages, or their equivalent, the ventral arches, in the 

 same space, and the break between the posterior margin of the py- 

 gidium and the appendages is less than in the preceding sections, but 

 more than would be taken by the three or four missing appendages. 

 From these facts it is considered that there is one pair of appendages 

 to each segment, and it is so expressed in the restoration on Plate VI. 

 fig. 1. In enumerating the number of arches or pairs of appendages 

 the cephalic appendages have not been included, as tliere is still some 

 uncertainty as to the number of appendages appertaining to the head. 

 The four pairs described are probably all that existed, and from the 

 marked similarity between the cephalic appendages of Limulus, Eu- 

 rypterus^ and the Trilobite we should naturally anticipate failure in 



