50 AFFINITY OF THE TRILOBITES 



two forms. It must l)e evident to every naturalist, that to recognize the analogues of the 

 genera Ci/pris or Ci/lhere in these shells, is far less probable than the affinity now suggested. 

 The more recent forms obtained from the fresh-water limestones of the Weald, and described 

 as Cyprisfnha, may, however, have really belonged to this genus. 



To prove by a priori reasoning that Euri/pterus was a shell-less Trilobite, just as 

 IhancUpus is a shell-less Phyllopod, may appear difficult ; yet even in this view I think I am 

 fully justified. The figures which Harlan has recently given us in his ' Medical and Physical 

 Researches,' p. 298, leave no doubt in my mind on the subject ; and it is from the study 

 of these figures, and from the similar one given by G. Fischer (Notice sur I'Eurypterus de 

 Podolie, Moscow, 1 839), that I have derived my conclusions. The animal, according to the 

 description, possessed a head which appears broader than the glabella of the Trilobites, 

 because it was softer and compressed, but otherwise corresponds with it in form. We 

 recognize in it two large lunate eyes, in which the black pigment of the centre may be very 

 well distinguished from the glassy spheres and lenses extended above it, as the figures of 

 Harlan distinctly show. These eyes were also unquestionably compound, and had a simple, 

 smooth, horny membrane. Three pair of organs seem to be affixed to the lower surface of 

 this head ; two of them being somewhat small, and situated at the anterior margin, and the 

 articulation of which is no longer recognizable, although the long bristles with which they 

 were furnished appear quite distinct. 1 take these for the antenujE, and suppose them to 

 correspond with the first two organs of locomotion of the Phi/Uojioda. The third pair of 

 organs of locomotion of the head were longer than the two others, thicker, more distinctly 

 articulated, free from bristles, but furnished at the end with hooks ; they probably formed 

 the accessory pair attached to and forming part of the mouth, and were useful to the animal 

 when seizing its prey. It decidedly appears, from Figure 2 of Harlan's Plate, that there 

 followed behind these three pair, and at the first thoracic ring, a couple of large, broad, 

 articulated, but soft fin-feet, the number of joints of which appears to have been five. 

 These also undoubtedly bore bristles at their margin, but their delicate nature prevented 

 the impression from being retained. Together with this first thoracic ring, I recognize 

 in Harlan's figure (Fig. 1) twelve rings, although in Fig. 2 there are only ten distinctly to 

 be seen, but in this case the extremity of the abdomen is injured ; Fischer has represented 

 fourteen rings, and a still further number is indicated in his figure. This impression, indeed, 

 seems generally to point to specific differences, on account of the sharp lateral prongs of the 

 rings of the body ; but we might also take these lateral lobes for the extreme ends of 

 the fin-feet, and assume at the same time that they were much smaller than the first 

 pair, according to the analogy of Jjms (Table VI, Fig. 1). I am myself decidedly of this 

 opinion, and consider that it is not feet that are visible in the figures of Harlan and of 

 Dekay, the animal having been too much compressed by the petrified mass during its 

 inclosure, to admit the possibility of the extreme ends of the feet projecting in this manner. 

 The softer abdominal side of the body, together with the feet, may, however, have been 

 already cast off in these very specimens, and this is another and also a very probable 

 conjecture. I believe in other respects that of the whole number of the rings we must 

 reckon nine as belonging to the real thorax, the remainder being abdominal. The great 

 diminution of the body from the ninth ring, and the equal breadth of the succeeding rings 

 arc reasons in favour of this conjecture, and the deviating formation of these first rings in 



