22 VISIBLE STRUCTURE 



The temporal sutures in the first case, by no means pass over to the lower side of the 

 cephalic shield, but continue at the anterior margin of it, and there meet together, so that 

 both are only the different directions of one suture going towards the left and the right. 

 This formation I observed in Oqi/f/ia, Pliacopfs, Amphits expanms, Wahl., and in all flat-headed 

 species of this genus, as far as I have been able to investigate them. This transition takes 

 place in the shape of an arch terminating near to the anterior margin in Amphus cxpaiisus, 

 A. laviceps, and A. {Nilens) armadillo, as also in Oyyyla and Phacops; it takes place, however, in 

 A. ramiceps, A. anpistifrons, and A. extemiatus, in a sharp, more or less pointed, angle. I could 

 never recognize a suture proceeding from this point, which had divided the lower surface of 

 the shell, v*^ith any degree of certainty ; Pander, however, has found such, and considered it 

 as the line of separation of his side branchiae {vide Table IV, B, of his work). The entire 

 cephalic shield of this pointed headed Ampjlnis likewise therefore only consists of three pieces, 

 an upper internal one, which covers the cephalic pi'otuberance, and which I term central 

 shield {scutum centrale), and two upper external ones, which at the same time pass over to and 

 form the lower side, as far as we are acquainted with it. I call them margin shield {scuta 

 marginalid), or temporal shields {scuta temporalin). In the second case the two temporal sutures 

 extend themselves over the anterior margin of the head, and reach, separately, that lower 

 margin of the cephalic shield which incloses the region of the mouth, and which I shall 

 subsequently describe, separating it from the cephalic shield. The anterior end of the central 

 shield in this case, therefore, also passes over to the lower side, and thus we have three 

 shields of the head-crust, a simple central shield, and two margin shields. The Oleneides 

 belong to this group. The central shield in these only occupies the central part of both 

 margins, and the entire lateral portions complete tlie marginal shields. The two temporal 

 sutures, in the third case, also terminate quite separately, reach exactly the angle of the 

 head-crust at the posterior part, but are connected anteriorly beneath the protuberant 

 margin of this crust by a transverse suture, which here separates a piece of the lower 

 surface of the shell placed anteriorly to the region of the mouth, so that four pieces are thus 

 formed, viz. a central shield, two marginal shields, and a shield situated in front of the mouth, 

 which I term scutum rostrale, and the suture which separates it I also term sutura rostrcilis 

 Such a structure may be met with in Cahjmene and IllcEnus* 



Such is the account I have to offer concerning these sutures of the head-crust ; I have 

 only further to observe, that similar unions of the pieces of the crust, by means of sutures, can- 

 not be traced in any of the existing Crustacea, but are only found in true insects of the present 

 W'Orld ; they constitute, therefore, a most remarkable and important peculiarity of the Trilobitcs. 

 We shall see subsequently that they do not occur again at any of the other segments or shields 

 of the shell of the Trilobites. No satisfactory conjecture as to their purpose can indeed be 

 hazarded without an accurate observation of living animals. Pander's opinion (p. 117), 

 " that the connexion of the parts is perfectly dissolved by this suture," and that in the living 

 state of the animal it had served for the purpose of removing the lateral shields from the 

 central, and thus permitting a changeable distance of both from one another, at the option 

 of the animal, can scarcely be founded on fact, for at the present day we by no means find 



* Professor Löven observed in the genus Trinucleus or Cryptolitlius a new and very different type 

 marked by the course of the sutures, which I shall describe in giving the character of the genus. 



