Prof. H. H. Swinnerton — Classification of Trilobites. 491 



Walcott 1 places his new genus Mollisonia near the Agnostidse, and 

 in describing it refers to indications of eyes and facial sutures on the 

 dorsal side. The course of the sutures is not quite clear either from 

 descriptions or figures, but they seem to conform to the proparian 

 type. The presence of seven free segments gives the deathblow to 

 Jaekel's 2 suggested divisions Miomera and Polymera. At the same 

 time it supplies strong support for his view 3 that the Agnostidse are 

 highly specialized Trilobites. This specialization in so many respects 

 supplies further support for the hypothesis that their blindness is not 

 primitive, and that the absence of facial sutures is due to fusion and 

 not to the submarginal position of the cheeks. 



While the orders Opisthoparia and Proparia correspond to facts 

 easily and clearly observed in nature the same cannot be said of the 

 order Hypoparia. The existence of a hypoparian stage in develop- 

 ment and in evolution is open to question. Beecher himself does not 

 seem ever to have actually seen free cheeks on the under side either 

 in young or adult forms except in the doubtful instance of the 

 AgnostidaB. In fact, there seems to be no case known of an adult 

 Trilobite which is undoubtedly hypoparian. 



As already seen, Beecher based his conception of a hypoparian 

 stage in evolution largely upon the development of both opistho- 

 parian and proparian forms. He did not, however, give adequate 

 consideration to either the development or the adult structure of that 

 most primitive of all Trilobite families, the Mesonacidse. Before 

 considering them it is necessary to emphasize the importance and 

 value of one of Beecher's 4 fundamental conceptions, viz. that in the 

 Trilobite organization the facial sutures 5 when present are essentially 

 associated with the eye. Even if the presence of a visual area be 

 denied 6 this conception is still true for the eye-lobe or the homologue 

 of the palpebral lobe. 



When Walcott 7 instituted the family Mesonacidse he described it 

 as being distinguished from the Paradoxidae by the absence of facial 

 sutures. Beecher 8 states that in Olenellus and Holmia " false sutures 

 are recognized" which are "evidently real sutures in a condition of 

 symphysis, which often occurs in Phacops ", etc. This explanation 

 — loss by fusion — was the only one open to him since he believed 

 that even in the earliest stages of evolution sutures were present. 

 Some years later Walcott 9 in discussing the sutures of the family 

 says, " facial sutures are rarely represented even by elevated lines on 

 the exterior surface or depressed lines on the interior surface of the 

 cephalon." He also in defining the family afresh says "facial 



1 Smithsonian Miscellaneous Collections, vol. lvii, p. 195, 1912. 



2 Vide supra. 



3 Op. cit., p. 392. 



4 Op. cit., pp. 100, 191. 



5 Cf. J. Barrande, op. cit., pp. 615, 616. Barrande decided that this 

 association of sutures with eyes was not essential. He based his view especially 

 upon the existence of a marginal suture in Trinucleus. 



6 Woods, op. cit., p. 233. 



7 Tenth Ann. Bep. U.S.G.S., 1891, p. 635. 



8 Op. cit., p. 191. 



9 Smiths. Misc. Coll., vol. liii, p. 242, 1910. 



