MUSEUM OF COMPARATIVE ZOOLOGY. 205 
The mode of attachment of the leg to the ventral surface is shown in 
the transverse section for Calymene in Plate III. fig. 9, and in Cerau- 
rus in Plate IL fig. 3. The longitudinal section is given in Plate V. 
figs. 1 and 3, for Calymene, and in Plate IL. fig. 6, for Cerawrus. 
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 pygidium 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 Vik 
fig. 1. In enumerating the number of arches or pairs of appendages 
the cephalic appendages have not been included, as there 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, Fu- 
rypterus, and the Trilobite we should naturally anticipate failure in 
