Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/666

 from the space between the two lobes of the hypostoma, where we must suppose the mouth to he situated, backwards along the median line to the pygidium. This corresponds in position to the sternum of the ordinary Crustacea. The legs are arranged in eight pairs, the bases of each pair being situated exactly under one of the eight segments of the thorax, and at the sides of the sternal groove.

The legs of the first pair are better-preserved than the others. They curve forwards and can be traced to a point nearly under the outer edge of the eye, or, rather, between the eye and the outside of the head. The other seven pairs follow at the average distance of two and a half lines from each other. The eight pairs thus occupy about twenty lines of the length of the ventral surface. This is exactly the length of the thorax, measured on the upperside. This trilobite has always eight segments in the thorax ; and there is thus on the underside one pair of appendages to each segment. Although some of them are very imperfect, and the portions that remain are somewhat displaced, with a little study of the specimen it can be seen that they all curve forwards, and are thus, most probably, ambulatory rather than natatory legs.

There appear to be several joints in each of these appendages; but the exact number cannot be made out. On the left side, the first four legs show very clearly that there are at least two, one at five lines from the side of the groove, and another about three hues further out. The position of each of these is indicated by a small protuberance (Pl. XXXI. fig. 1, n). On the right side the preserved portions of the legs are longer, and thus indicate a greater number of articulations, although they cannot be distinctly seen. I think that each leg consisted of at least four or five articulations.

On the pygidium there are three small ovate tubercles, arranged in a line, that seem to be organic (fig. 1, d) ; and if they are so, they are, perhaps, the processes to which respiratory feet were attached. The length of the specimen is four and a half inches, and the width two and a half inches. On a side view the height of the head just behind the eyes is nine lines, and at the middle of the thorax about seven lines. The depth of the internal cavity at the back part of the head is seven lines, and at the last segment of the thorax four lines. The plane in which the legs are situated is therefore not so low down as the extremities of the pleura;. The visceral cavity is thus about one-third less than the whole bulk of the animal (Pl. XXXII. fig. 1).

The above is all that I desire to say at present concerning this remarkable specimen. The first and all-important point to be decided is, whether or not the forms exhibited on its underside are truly, what they appear to be, locomotive organs. If this question be decided in the affirmative, it will then remain for Carcinologists and others to homologize them with the limbs of existing Crustacea. It is scarcely necessary to remark, in this place, that, in view of the great zoological questions that arc at present being discussed, the correct determination of the affinities of the Trilobites is of extraordinary importance.