NO. 3 UPPER CAMBRIAN TRILOBITE FAUNAS RASETTI 75 



( 1 ) The width of the palpebral area relative to the glabellar width 

 at the same level. 



(2) The sagittal length of the frontal area relative to the length 

 of glabella, including the occipital ring. 



(3) The relative sagittal lengths of preglabellar field and border. 

 This ratio and the preceding one, although of undoubted statistical 

 significance, were found variable within a population for some of the 

 species (e.g., A tarda, A. camiro). Such variations appeared uncor- 

 related with either stratigraphic position or size of the cranidia. 



(4) The (exsagittal) length of the palpebral lobes relative to the 

 length of glabella inclusive of occipital ring. 



(5) The (exsagittal) length of the palpebral lobe relative to the 

 exsagittal distance between the posterior end of the palpebral lobe 

 and the posterior cranidial margin. 



(6) The (transverse) width of the posterior area relative to the 

 width of the occipital ring. 



(7) The angle of divergence of the anterior sections of the facial 

 suture and the more or less sharp change of direction of the suture 

 in crossing the anterior border furrow. The posterior section of the 

 facial suture has the same course in all species. 



(8) The slope of the palpebral area, also a somewhat variable 

 feature within a population. 



(9) The slope and convexity of the preglabellar field and the 

 more or less sharp angle formed by the border with respect to the pre- 

 glabellar field. These features show large intraspecific variability, 

 e.g., in A. laxa and A. tarda. 



(10) The free cheeks in some cases offered useful characteristics 

 in the features of the border and genal spine. 



(11) The pygidia may be virtually indistinguishable for several 

 species. In other cases species whose cranidia are very similar may 

 show quite different pygidia. 



All the species previously described from Virginia and Tennessee 

 by Walcott (1916a) and Resser (1938a) could be found again, repre- 

 sented by more abundant and better preserved material, and with 

 three exceptions are discussed herein. Saratogia aruno Walcott and 

 Clevelandclla nitida Resser are specifically undetermined forms of 

 Aphelaspis possessing an occipital spine, presumably identical with 

 either A. arses or A. arsoides. Aphelaspis hamhlenensis is based on a 

 complete exoskeleton flattened and weathered in shale ; since individ- 

 uals so preserved do not preserve the specific features, the name should 

 be restricted to the holotype. In these three cases the stratigraphic 

 position of the type specimens is unknown. 



