WATERLIME GROUP. 
399 
tions, it appears to be free from attachment to the adjacent parts, and would appa¬ 
rently leave beneath it a communication with the internal economy of the animal. 
This appendage has an articulation below the margin of the body segment to which 
it is attached; and at the extremity of this joint are articulated two terminal 
processes, giving considerable freedom of motion to this part*. 
The under part of the carapace with its organs, including some of the first seg¬ 
ments of the body, is represented in the figure on the following page. This diagram 
is constructed nearly of the size of the largest specimen of E. lacustris. The relations 
of all these parts are demonstrable in several individuals. 
In the process of compression, the organs surrounding the mouth are flattened, and 
more or less displaced in nearly all the specimens. In their natural relations, the 
rounded bases of the first four pairs of feet are more nearly over one another, and 
the mouth has a more vertical or less elongate form; while in the figure, in order 
to conform to the actual conditions, but more in order to show the parts clearly, 
they have been drawn in this manner, so that the mouth has twice the length that 
otherwise it would have. As the feet lie one partially above the other, the bases of 
the fourth pair of feet are covered by the first joint of the fifth pair, as far as the 
ridge shown upon the long second joint; while the bases of the third pair are, in 
like manner, covered by the fourth. 
* This organ bears so much resemblance to the membranaceous feet of Limulus, that I introduco 
in this place a figure of the same. In Griffith’s edition of Cuvier’s Animal Kingdom, Vol. xiii, 
page 362, in describing Limulus, the following language is used : “ The last two feet of this 
“ buckler are united, and in the form of a large membranaceous leaflet, almost semicircular, sup- 
“ porting the sexual organs at its posterior face, and presenting in the middle of an emargination of 
“ the posterior edge two small triangular divisions, elongated and pointed, which appear to represent 
“ the internal fingers of the forceps. Some sutures indicate the other articulations.” 
Fig. 4. The membranaceous feet referred to above. The 
letters correspond to those used in the figure of the locomo¬ 
tive appendage of Euryptervs, for parts supposed to have a 
similar relation. 
Fig. 4. 
The similarity of form and relative position of the appendages in Eurypterus is at once obvious, 
and we may suppose that the organ has performed similar functions. 
