The Facial Suture of Trilobites. 107 



discussion to this case it should be noted that the region which he 

 calls the cephalon in this species has hitherto been unanimously 

 regarded as the pygidiura. The only reason he advances for 

 disagreeing so utterly with previous workers is that he has discovered 

 structures like free cheeks on the ventral side of the "pygidium". 

 "Until he brings forward independent evidence to prove that the 

 pygidium is not a pygidium, or that the ventral structures are free 

 cheeks, his discovery cannot be regarded as valid. Meanwhile, all 

 independent evidence seems to be against him, and his discovery 

 seems to prove only that Agnostus underwent ecdysis by way of the 

 pygidium. 



It is of course possible that Raymond may yet prove his case. 

 When he does that his further deduction 1 will be valid, viz., that 

 the suture and ventral linear "free cheeks" he has found in 

 Agnostus are "analogous" with the marginal suture and detachable 

 doublure of Pcedeumias. But this can only prove that they cannot 

 be homologous with the dorsal facial suture and dorsal free cheeks 

 of other trilobites. 



The fact that the closely allied but distinctly more primitive genus 

 Pagetia 2 has both eyes and true facial sutures indicates that 

 Agnostidse have lost both these features, and have reverted 

 secondarily to a marginal ecdysial line. This point of view receives 

 support, on the one hand, from a consideration of the extraordinary 

 degree of specialization attained by the Agnostidse in all respects, 

 and on the other hand from the existence of indications of eyes and 

 true facial sutures in the allied but much more primitive genus 

 Mollisonia? 



The Ecdysial Line in the Trinucleid^;. 



The problem of the facial suture in Trinucleus has been very fully 

 discussed by Reed, 4 who concludes that the true facial suture has 

 disappeared, that the free cheeks have fused with the fixed cheeks, 

 and that the suture which is present along the margin has come into 

 being secondarily. Raymond disagrees entirely with these con- 

 clusions, and regards the marginal suture and ventral free plates of 

 Trinucleus as facial suture and free cheeks respectively. 5 He 

 rightly homologizes these features with those which he claims to 

 have discovered in Agnostus 6 and, by implication, with the suture 

 and free doublure of Pcedeumias. In other words Trinucleus has 

 nothing which is homologous with the true facial sutures and free 

 cheeks, and Reed's conclusions are proved to be correct by Raymond's 

 own evidence. Moreover, though Eeed finds no satisfactory traces 

 of dorsal facial sutures, he does find eye-lines and vestiges of eyes. 

 "When these occur they do not show that association with the 

 marginal suture which would prove this to be a true facial suture. 



Orometopus, if it be related to the ancestral stock of Trinucleus, 



1 Ibid., p. 208. 



2 C. D. Walcott, Smiths. Misc. Coll., vol. Ixiv, No. 5, p. 407, 1916. 



3 Smiths. Misc. Coll., vol. lvii, p. 195, 1912. 



4 Geol. Mag., 1916, p. 175. 



5 1917, pp. 201 et seq. 



6 p. 203. 



