The Facial Suture of Trilobites. 109 



protaspis of certain trilobites are therefore not to be regarded as 

 ancestral features, but, more probably, as adaptive characters 

 associated with a planktonic habit. 1 



If this be the true interpretation of the facts, then the backward 

 shifting of the eye during development has no phyletic significance, 

 but is merely associated with the cessation of the larval planktonic 

 mode of life and the assumption of the benthic habits of the adult. 



Beecher was unfortunate in his choice of developmental evidence 

 upon which to base his opinions. Had he given more attention to 

 the larvae of the Mesonacidae he would never have instituted the 

 division Hypoparia. These larvae retain traces of several pleurae in 

 the cheek region, and thereby show themselves to be the most 

 primitive of all known trilobite larvae and therefore the most 

 valuable indicators of ancestral conditions. But even in the 

 youngest of these larvae known the eyes are dorsal and the facial 

 suture is absent. This agrees with the evidence of the comparative 

 study of the adult and therefore outweighs the perhaps more 

 abundant evidence from more specialized larvae. 



Raymond falls into the same error 2 as his great teacher when he 

 hints at the resemblance between the young of Trinucleus and the 

 adult of Agnosius as evidence of affinity. A trilobite with such 

 forms as Harpes and Dio?iide closely allied to it could not possibly 

 have descended from an Agnostics-like ancestor. 



The Trilobita are Monophyletic. 

 Raymond finds difficulty in accepting my interpretation 3 of 

 Mesonacid structure and development because it implies that "the 

 anterior segment was not oculiferous " i in them as in other trilobites, 

 and that therefore trilobites must be polyphyletic. Why he should 

 consider it necessary that the oculiferous segment must be the 

 anterior. one he does not state, but as long as there are to be found 

 annelids such as Hirudo having eyes on every segment as far back as 

 the fifth there is no need to postulate the first segment as the only 

 one that can be oculiferous in trilobites. It has been shown above 

 that the ocular segment is the sixth from the posterior margin in all 

 trilobites which exhibit sufficiently abundant traces of segmentation 

 to enable an opinion to be formed. Whether this is the anterior 

 segment or not does not matter; the essential point is that the 

 segment which bears the eye is the same for all trilobites, so that 

 from this standpoint the order is monophyletic. 



Conclusion. 

 The trilobites are a compact group, the members of which at first 

 underwent ecdysis along a line which may be called the marginal 

 suture. " To facilitate the removal of the covering of the eye 

 in moulting" 5 dorsal facial sutures appeared independently in 

 several distinct lines of descent. It is necessary to emphasize the 

 fact that taken as a whole the true facial suture is composite, 



1 L. Dollo, La Paleontologie ethnologique, Bruxelles, 1910, pp. 406 et seq. 



2 p. 204. 



3 Geol. Mag., 1915, p. 492. 



4 Amer. Journ. Sci., 1917, p. 208. 5 Ibid. 



