Prof. H. H. Swinnerton — Classification of Trilobites. 489 



According to the definition of the last-named family there are 

 " small free cheeks visible upon the dorsal surface". 1 If figures are 

 to be trusted the members of this family are as typically opisthoparian 

 as are Conocoryphe and Atops. Presumably they are placed here 

 because of their evident relationship to the Trinucleidse. 



Among the Trinucleidse two lines have been described as facial 

 sutures, one which has only occasionally been found and which 

 pursued the course of an advanced opisthoparian type, viz. across 

 the cheek from the anterior of the glabella to the posterior margin. 

 The other ran along the margin of the cephalon. Barrande discussed 

 the relative merits of the two lines, and concluded that the former 

 was only an ornamental nervure and that the latter was the true 

 suture. During recent years evidence has accumulated which shows 

 that Barrande 2 was in error. 



The first piece of evidence is negative and suggestive rather than 

 conclusive. If the marginal line be the facial suture, then the dorsal 

 portion of the broad marginal limb belongs to the fixed cheek and the 

 ventral to the free cheek. Numerous pits with connecting pores pass 

 through this limb from the upper to the under surface. These might 

 be expected to show traces of the separation between the free and the 

 fixed cheek regions. According to Reed, 3 "the hour-glass shaped 

 hollow pillars representing the opposite and communicating pits in 

 the upper and under surfaces are continuous and show no transverse 

 plane of fission." It is of course conceivable that secondary fusion 

 has taken place, but why should there be fusion in the walls of the 

 pits and not along the edge of the cephalon ? 



The genus Orometopus, which possesses well-formed compound eyes 

 and a clearly marked suture of an advanced opisthoparian type, 

 supplies positive and conclusive evidence. This may be best stated 

 in the words of Lake 4 : Orometopus "is the earliest genus of the 

 Trinucleidas and must therefore be looked upon as the primitive 

 form ; and the conclusion is inevitable that in the later genera the 

 absence of compound eyes and supposed marginal position of facial 

 sutures are degenerative characters. It is indeed no longer improbable 

 that the ocelli which occur in the middle of the cheeks in some species 

 of Trinucleus and Ampyx may represent normal eyes, and that the 

 lines which Salter and M'Coy observed running from these ocelli to 

 the margins may be fused facial sutures as they supposed. If this be 

 so the Trinucleidse can scarcely be included in Beecher's Hypoparia ". 



Dollo 5 regards Trinucleus as a Trilobite adapted for a life spent 

 permanently in the mud. If this was its mode of life it is easy to 

 understand the reduction of the compound eye to such a simple one 

 as that of Tretaspis or of the young Trinucleus. The loss of the 

 facial suture by the fusion of free with fixed cheeks would produce 

 a more rigid head-shield like that of other Trilobites said to have had 

 a burrowing habit such as Olenellus and other Mesonacidas. The 



1 Textbook of Paleontology, Zittel, ed. Eastman, vol. i, p. 713, 1913. 



2 J. Barrande, Systeme Silurien, Trilobites, p. 616. 



3 Geol. Mag., 1912, p. 347. 



4 Monogr. Palaeont. Soc, 1907, p. 45. 



6 La Paliontologie Ethologique, Bruxelles, 1910, p. 417. 



