APPENDIX. 75 



And first I would observe, in reference to the varied forms of Trinucleus, with 

 corrugated, punctate, perforate, and serrated margins to the head-shield, that we 

 may see in the larval " king crab " or " horse-shoe crab " {Limulus polyphemus) of 

 North America just before hatching, that the head-shield retains indications, in 

 the well-marked fimbriated hepatic lobes, of the presence of a once divided border, 

 corresponding with the five or six distinct cephalic somites. This fimbriated 

 margin to the cephalic shield, seen in the young of Limulus, is observable also in 

 the head-shield of Hemiaspis limuloides from the Lower Ludlow of Shropshire. 

 As to the explanation given by M. Barrande for the series of depressions, or 

 invaginations, of the crust, of various Trilobites which are seen to correspond 

 with each thoracic somite, and are placed exactly in the depression of the furrow 

 on each side of the dorsal axis, and are even observable in the axal furrows of the 

 pygidium of Dalmannia, we agree with him entirely, and there can be, I think, no 

 doubt whatever that they are perfectly homologous with the similar pits or 

 indentations of the crust, observable in the thoracico-abdominal shield of 

 Limulus, which gives rise to six powerful calcareous processes within {Apodemata, 

 Milne Edwards ; Entapophyses, Owen) for the attachment of the muscles required 

 for the locomotory organs. 



But whether the two pits or pores placed one on each side of the glabella, in 

 front of the compound eyes, are of the same nature (as Barrande assumes them to 

 be) is an open question. That they are not the remains of Antennce, as supposed 

 by Prof. M'Coy, I think we may feel quite satisfied, as we know of no Crustacean 

 whatever with antennae arising from the dorsal aspect of the cephalic shield in the 

 manner supposed by M'Coy. 



The antennse are either on the margin of the head as in many Isopods (see 

 Plate IX, figs. 8 and 9), or beneath as in Limulus, Apus, and other forms of 

 Entomostraca. 



Mr. John Young, F.G.S., of the Hunterian Museum, in the University of 

 Glasgow, a most careful observer, and to whom I am indebted for the loan of many 

 interesting specimens of Carboniferous Trilobites, several of which illustrate this 

 point in an admirable manner, suggests, in a note to me, that he has long thought 

 that these puncta might possibly prove to be ocelli. "Whether this be the correct 

 interpretation or not, at any rate it seems a more plausible hypothesis than that 

 suggested by Prof. M'Coy. 



It is true that such an association of larval eye-spots with compound eyes has 

 not as yet been met with in any of the Trilobita, but it occurs in the Merostomata, 

 both recent and fossil (Pterygotus, Slimonia, and Limulus), and its discovery 

 would be hailed as further evidence in favour of the undoubted close affinities of 

 both these groups with the Scorpionidce, and so with the Arachnida. 



We have noticed two instances of pores in the Isopoda, and it is probable that 



