Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/677

 Fig. 1. — Buccal organs of Asaphus platycephalus.

p- m- h, hypostoma ; p, palpus ; m, maxilla?

I was at once attracted by a specimen of Asaphus, from the Black Trenton Limestone (Lower Silurian), which has been much eroded on its upper surface, leaving the hypostoma, and what appear to be the appendages belonging to the first, second, and third somites, exposed to view, united along the median line by a longitudinal ridge. The pseudo-appendages, however, have no evidence of any articulations. But what appears to me to be of the highest importance, as a piece of additional information afforded by the Museum specimen, is the discovery of what I believe to be the jointed palpus of one of the maxillae (Fig. 1), which has left its impression upon the side of the hypostoma — just, in fact, in that position which it must have occupied in life, judging by other Crustaceans which are furnished with an hypostoma, as Apus, Serolis, &c.

The palpus is 9 lines in length ; the basal joint measures 3 lines, and is 2 lines broad, and somewhat triangular in form.

There appear to be about seven articulations in the palpus itself, above the basal joint, marked by swellings upon its tubular stem, which is 1 line in diameter.

There can be no reason to doubt that the Trilobita possessed antennules, antennas, mandibles, maxillae, and maxillipeds, as we find the same organs preserved in Crustacea of equal antiquity (e.g. Slimonia acuminata and Eurypterus remipes, both Upper Silurian forms). "With regard to the "Panderian organs" mentioned by Mr. Billings, T venture to suggest that the small circular impressions, seen upon the pleurae of many Trilobites are only the fulcral points upon which the pleuroe move, and correspond to the ball-and- socket joints which mark the limbs and segments of all the higher Crustacea.

We know of no Crustaceans having two pairs of appendages to each segment ; but it is characteristic of Crustacea to have their appendages bifid, giving rise to an endopodite and an exopodite ; but these are always given off from a common base.

Having regard to the characters presented by the Trilobita as a group, we should be inclined to place them near to, if not actually in, the Isopoda Normalia.

In all this group, the branchiae are abdominal, being placed under the broad and well-developed pygidium, which is not equivalent to the telson of the higher Crustacea, but is composed of several segments soldered together, in fact, representing the true abdomen. It is here, then, we should expect to place the branchiae in the Trilobites, and not upon the epimera of the body-segments.

If any objection should be urged against the organs observed in the specimen in the British Museum being really legs, I would suggest that they may be considered good evidence of the presence of