10 



STRUCTURE AND HABITS 



I am indebted to Mr. Spence Bate for some friendly criticisms of these views, and 

 shall wait with interest for his promised memoir on the ' Homologies of the Trilobite, 

 and its Habits.' 



The general structure of the animal will be best perceived by referring to the woodcuts 

 a little further on ; and while there is, in the greater part of this structure, a sufficient 

 resemblance to the ordinary Crustacea, there are one or two points in which the Trilobite 

 differs from all other groups, and they happen to be obvious ones. 



The curious so-called facial suture, a line of division which is only faintly indicated in 

 the Limulus, and which has, perhaps, no other representative 1 in the whole Crustacean 

 class, sufficiently distinguishes the Trilobite. It divides the head into two portions, an 

 anterior one that bears the eye, and a posterior that covers the stomach. The latter 

 segment is much larger than the former, and may be formed of several rings. 



And then there is the " trilobation." Whatever tendency some of the higher 

 Crustacea may show to this, and in whatever degree a few of the Trilobites may lose it, 

 it is the conspicuous character of the whole order, and has, doubtless, an important 

 meaning. Limulus also shows a trace of this trilobation ; but it is accidental, rather than 

 characteristic, in other groups. 



I give here a copy of the original figure in the ' Memoirs of the Geol. Survey/ vol. ii, 

 Part I, p. 334. It is not drawn from any particular Trilobite, but is a general expression 

 of the structure. 



And I have added one or two terms from Barrande's more complete figures. 



Fig. 1. 



Fig. 2. 



Beginning with the head, or carapace (Fig. 1, upper side ; Fig. 2, under side), we 

 recognize the following portions: 



Glabella (A), bounded by the axal furrows (A*), and including the neck-furrow (c) ; 



1 I am bound to state th.it Mr. S. Bate believes he has discovered the analogue of this suture on the 

 under side of the crab and lobster ; he finds it too in Argulus, &c. But his views are not yet fully 

 published. 



