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



sutures rudimentary or in a state of synthesis " } The rest of his 

 monograph leaves it uncertain as to whether or not he is really using 

 the word rudimentary as opposed to vestigial and synthesis as 

 opposed to symphysis. If the definition be taken at its face value he 

 is evidently leaning towards the point of view of the present writer, 

 viz. that the facial sutures were not necessarily present in primaeval 

 Trilobites and that the Mesonacidse exhibit the Trilobite organization 

 just when these lines are coming into being. The genus Nevadia 2 

 (Fig. 2a), which is one of, if not the most primitive of undoubted 

 Trilobites, and which should therefore approximate to Beecher's 

 hypoparian archetype, has no definitely established facial sutures, and 

 its eyes, like those of all other Mesonacidse, are situated on the dorsal 

 side far from the margin of the cephalon and close to the glabella — 

 that is to say, it is the reverse of hypoparian in every respect. 



Not less decisive is the evidence supplied by the development of 

 the Mesonacidse. Beecher 3 infers that, because the eye travels over 

 the margin on to the dorsal side during the development of the types 

 he considers, therefore the free cheeks must be wholly ventral. The 

 earliest known stages in the development of Olenellus {JElliptocephalus) 

 asaphoides* are quite as early as any of those referred to by Beecher 

 in other types, nevertheless from the very outset the rudiments of 

 the eyes are dorsally situated and are well within the margin of the 

 head-shield. This fact implies that if facial sutures had been present 

 the free cheeks would have been upon the dorsal side. Facial 

 sutures, however, are not present, even though the embryo is so 

 primitive that, unlike any known embryo outside this family, it 

 shows the pleural elements of the fixed cheek region quite distinctly. 

 According to Bernard 5 the facial suture is formed in Trilobites 

 generally along the line which separates the first and second pleurae. 



Walcott 6 has described a new Crustacean genus Marella in which 

 he says "the trilobite is foreshadowed". Though so beautifully 

 preserved that even minute details in the structure of the appendages 

 may be discerned there is no mention in his description nor indications 

 in his plates of a facial suture. 



Along with Marella Walcott 7 also described a primitive Trilobite, 

 Nathorstia transitans, which, whilst it is "essentially a Trilobite", 

 yet exhibits characters which link it to the Branchiopods and Mero- 

 stomes. The position of the eyes and the absence of facial sutures in 

 this genus also bear out the conclusions already indicated above that 

 there was no such stage as the hypoparian in the evolution of 

 Trilobites as a whole. On the contrary, the earliest trilobitic organisms 

 had no facial suture. The Branchiopods and the Merostome& 

 approach more closely to these than do any other animals. In 

 the former there are no traces of sutures. In some of the latter 



1 Smiths. Misc. Coll., vol. liii, p. 236, 1910. 



2 Ibid., p. 257. 



3 Op. tat., p. 100. 



4 Ford, Amer. Journ. ScL, 1877, 1881 ; Walcott, op. cit., 1891, pi. Ixxxviii; 

 Bull. U.S.G.S., No. 30, pi. xvii, 1886. 



5 Q.J.G.S., 1894, p. 419. 



6 Op. cit., 1912, p. 192. 



7 Op. cit., p. 194. 



