108 Professor H. H,. Swinnerton — 



supplies proof positive that Reed is right. Raymond, 1 however, 

 denies the relationship because Orometopus has compound eyes, large 

 free cheeks on the dorsal side, less specialized glabella, more than 

 six free thoracic segments, and a square hypostome. The first two 

 differences, being the subjects of dispute, may be left out of account. 

 The remaining differences are not differential characters, but are 

 such as are usually looked for between the earlier and later members 

 of a genetically related series. The type of argument he thus 

 applies to Orometopus would prove that Mceritherium had no relation- 

 ship to Elephas, or Hyracotlierium to jEquus. 



Incorrect Usage of Embryological Evidence. 



Agnostus and Trinuchus are the typical representatives of 

 Beecher's Sypoparia, which was founded upon inferences drawn 

 from the study of the development of many trilobites. In this it 

 was observed 2 that the eye appeared most frequently upon the 

 margin, and travelled backwards bringing the facial sutures and free 

 cheeks with them as development advanced. The occurrence of 

 this phenomenon in so many trilobites was taken to indicate that the 

 ancestors had marginal eyes, marginal or sub-marginal facial sutures, 

 and ventral free cheeks. 



In coming to this conclusion sufficient care was not taken to test 

 the value of the developmental evidence by a comparative study of 

 the adult structure. The larvae to which Beecher attached most 

 importance have, in addition to the marginal position of the eyes, 

 a glabella which increases in calibre anteriorly and often extends to 

 or even beyond the front margin of the head-shield. Compare this 

 condition with that found in such primitive adult trilobites as 

 Conocoryphe, Ptychoparia, etc., which were known to Beecher, and 

 Nevadia* and Nathorstia, 1 which have been discovered since he 

 finished his work. In these adults the eyes when present are 

 dorsal in position, the glabella diminishes anteriorly, and does not 

 approach the frontal margin. When the facts of embryology clash 

 thus with those of comparative anatomy the former must be 

 interpreted with extreme caution. 



The conditions prevailing in less primitive trilobites throws a 

 flood of light upon the conditions in these larvae. Thus an increase 

 in the width of the anterior segments of the glabella is frequently 

 exhibited by the more advanced members of a progressive series, 

 cf. Olenellus with Nevadia. It can only be concluded that the larvae 

 which exhibit this feature are specialized also, at least, in this 

 respect. Again, in such a series as Cheirurus, Splicer exochus, and 

 Deiphon the forward extension and inflation of the glabella are 

 accompanied by an assumption of other features, such as the 

 marginal position of the eyes and curious isolation of the pleurae, 

 which according to Dollo point to a planktonic mode of life. The 

 peculiarities of the glabella and the position of the eyes in the 



1 p. 203. 



2 Amer. Journ. ScL, 1897, p. 184. 



s C. D. Walcott, Smiths. Misc. Coll., vol. liii, p. 256. 

 4 Ibid., vol. lvii, p. 194. 



