﻿22.8 BRITISH FOSSIL CRUSTACEA. 



fore be more than a supposition that there is any very close connection between a young 

 Trilobite, such as those represented on PI. XXXII, and a Limulus, such as we have 

 pictured to us by Dr. Packard (PI. XXXIII, figs. 1 — 5), possessed of a head-shield 

 affording evidence of at least six (if not of seven segments), and a hinder body (repre- 

 senting according to my view the thoracico-abdominal series) composed of nine segments 

 from its third or fourth moult within the protodermal egg-covering even before the 

 absorption of the yolk takes place. 



2nd. As to the position of the eyes, Dr. Packard observed " the compound eyes as 

 two white dots on the third segment of the cephalotJiorax (cephalon, H. W.). 



"The ocelli (he states) appear still later on in the mandibular?/ 1 or first segment. It 

 is thus proved that the eyes are not in Limulus developed on distinct segments " (op. 

 cit., p. 165). To this I would reply that at this stage most probably, as in some other 

 Entomostraca, the compound eyes are really concealed beneath the head-shield. The 

 ocelli, he states, are on the first segment ; but this is not " the mandibular segment " (see 

 pp. 4 and 5 of this Monograph, where, according to Bell, Dana, Bate, and Huxley, 

 the ophthalmic segment is the first, the antennary 2nd and 3rd, and the mandibular 

 the 4th). But of far higher importance in deciding this point is the position of the 

 nerves in connection with the supraoesophageal ganglion, as shown by Professor Owen, 

 whose plates I am permitted to reproduce (see PL XXXIV — XXXVI). 



Professor Owen remarks "The first pair of nerves is the 'ocellar' (PI. XXXVI, 

 a, a 1) ; the second pair of nerves is the ' ocular' (ib., n A)." Surely if the supposed 

 want of two pairs of antennal nerves taking their rise from the front of the supra- 

 oesophageal ring was sufficient to induce Dr. Dohrn to wish to remove the Lim.idi from 

 the Crustacean class, the position of the optic nerve ought to have some weight in 

 deciding the segment to which the eyes must really be referred. 



3rd. I believe I have almost in every case (save the one pointed out by Dr. Packard 

 in Part I, p. 39) spoken of the post-oral plate as the metastoma, not as the labium. Dr. 

 Packard in comparing the Merostomata with the Trilobita (op. cit., p. 190) — a com- 

 parison which I cannot consider as a happy one 3 — speaks of the metastoma of Eurypterus 

 and Limulus as the hypostoma, and compares it with the hypostoma of the Trilobites with 

 which it clearly has nothing in common ; since the hypostome of the Trilobite is always in 

 front of the mouth as in Apus, attached to the frontal doublure of the head-shield ; whereas 

 the great cordiform lip-plate or " metastoma ' of Pteryyotus, and the ' chilaria ' (Owen) of 

 Limidus are placed invariably behind the circumoral appendages, closing effectually the 

 posterior part of the buccal orifice. 



4th, There being, according to Dr. Packard's view, no distinct optic segment in 

 JAmulus, &c, he treats the first segment as antenna]. 



1 Possibly this may be a misprint, as elsewhere he speaks of it as the antennary segment. 



2 The reader's attention is drawn to the characters of Pterygottjs and Trilobita given at pp. 22b'-7. 



