WATERLIME GROUP. 395 
ceous feet behind the swimming feet of Limulus ; and, instead of being 
double, is anchylosed as in young Limulus. 
Since my comparisons, during investigations of these fossils, had been 
made almost entirely with Limulus , I am prepared to appreciate these 
views of Professor Agassiz*. 
The following is the description of the genus by Dr. Dekay : 
“ Caput a thorace non distinctum. Os ignotum. Oculi duo, sessiles* 
distantes, lunati. Abdomen elongatum, posticam versus extremitatem 
sensim gracilius, segmentis transversis subimbricatis divisum. Pedes 
octo; duo utrinque antici branchiferi, duo utrinque postici maximi, 
omnes lamellosi.” 
The specimens examined enable me to extend this description, and the 
genus may be characterized as follows i 
Genus EuryptErus ( Dekay). 
Body ovato-lanceolate, broader in front, gradually attenuated behind, and 
terminated in a spiiiiform tail. Carapace on the tipper side entire, 
somewhat semioval, wider than long. Eyes two, retiiforin or oval, dis a 
tant, sessile, placed within the margin of the carapace : simple oculi- 
form tubercles, or cotneae, two, situated stibcentrally on the carapace. 
Thoracic and caudal portions composed of thirteen, joints : the first set 
of these, belonging to the thorax, are narrower 5 the others, constituting 
the caudal or abdominal portion, are subquadrate, except the terminal 
joint, which is prolonged into a sublinear or lanceolate triangular spine 
with serrated edges* 
The first thoracic articulation on the lower side is double, consisting of 
the first two joints of the back anchylosed in one, and showing the suture. 
A central locomotive appendage is attached to and proceeds from the 
upper part of the first joint; and dividing the lower half of the same, it 
* This volume having been in print for a long time, with the exception of the hiatus left for the Eury- 
pterus, its completion can no longer be postponed; but the descriptions with the illustrations will appear as 
a separate paper; for which Prof. Agassiz has promised me a preliminary or introductory note. 
