4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



TERMINOLOGY 



The terminology here employed for describing the trilobite exoskele- 

 ton is essentially the same as that proposed by Howell et al. (1947), 

 with the modifications listed below. 



Several trilobite students have pointed out the opportunity of re- 

 storing the term "fixed cheeks" to its original meaning of the entire 

 nonaxial area of the cranidium, instead of a portion thereof. Conse- 

 quently, the area designated as "fixed cheek" by Howell et al. is now 

 called "palpebral area" and is bounded by the dorsal furrow, the ocu- 

 lar ridge, the palpebral furrow, and a straight line running from the 

 posterior end of the palpebral lobe to the proximal end of the posterior 

 marginal furrow. The term "width of the palpebral area" hence re- 

 places "width of the fixed cheeks" as employed by Lochman 

 (1947), indicating the transverse distance between the dorsal furrow 

 and the palpebral furrow. This distance is often compared to the 

 width of the glabella measured at its midlength. The length of the 

 palpebral lobe is measured along a chord. 



The term "frontal area" indicates collectively the preglabellar field, 

 marginal furrow and border. 



Terms such as "wide," "long," etc. are qualified by the abbrevia- 

 tions "sag." (sagittal), "exsag." (exsagittal) and "tr." (transverse) 

 whenever the direction in which the measurement is taken might 

 otherwise result in ambiguity. 



The posterior limb is divided into a proximal portion, extending 

 from the dorsal furrow to the palpebral lobe, and the remaining distal 

 portion. The ratio of the widths (tr.) of the two portions supplies a 

 useful taxonomic character. The entire width (tr.) of the posterior 

 limb is compared to the width (tr.) of the occipital ring. The width 

 of the posterior limb is measured along a chord, not the vertical 

 projection. 



All descriptions refer to the upper surface of the test unless other- 

 wise indicated. 



SYSTEMATIC DESCRIPTIONS 



Superfamily PTYCHOPARIIDEA 

 Family PTYCHOPARHDAE Matthew, 1888 



The Lower Cambrian ptychopariids are notoriously difficult to 

 classify (Lochman, 1947; Rasetti, 1951). Study of the new material 

 further emphasizes intergrading of the established genera and the dif- 

 ficulty of adopting such stringent discriminatory generic characters 

 as proposed by Lochman (1947), since adherence to this method 



