NO. 7 PTYCHOPARIID TRILOBITES RASETTI 21 



Remarks. — The type species has been misplaced in several genera. 

 Luxella is ob\'iously a close relative of Onchocephalus, as evidenced 

 by an intermediate form illustrated herein (Onchocephalus, species 

 imdetermined Xo. 3), showing widening of the palpebral area, 

 lengthening of the frontal area, and increasing size of the palpebral 

 lobes. In Luxella lux these changes have reached a point where they 

 justify generic distinction. From these features, it is obvious that 

 Luxella resembles Inglefieldia, Amecephaltis, and Alokisiocare, and 

 may well be in the ancestral line of some of these trilobites. The 

 chief difference is in the shape of the posterior limbs, which in all 

 these genera have a longer, horizontal distal portion, whereas in 

 Luxella lux the distal portion is shoner and sharply turned down- 

 ward, indicating a thorax with pronounced geniculation in place of 

 the fiat pleura of the Amecephalus type. Hence the species is not 

 referred to Inglefieldia, which it othens-ise resembles except in the 

 greater width of the palpebral area. 



An additional species from Quebec is referred to Luxella. 



LUXELLA LUX (Walcott) 

 Plate I, figure 4; plate 6, figures 9-11 



Ptychoparia lux ^^"ALcoTT, Smithsonian Misc. ColL, voL 67, No. 3, pL 12, ng. 5, 



1917. 

 Kochina? lux (Walcott), Resses, Smithsonian Misc ColL, toL 97, No. 10, p. 33, 



193S. 

 Crassifimbra lux (Walcott), Rasetti, Smithsonian Misc. C0II-, toI. 116, No. 5, 



p. 82, 195 1. 



Available material. — The t}-pe lot includes the holotype and several 

 parat}-pe cranidia. Among several topotA.-pe cranidia collected by the 

 writer is the almost perfect one illustrated herein. 



Description. — Most of the characters of the species were m.entioned 

 in the generic diagnosis and are not repeated. The glabella shows 

 two very faint pairs of furrows at the sides. Occipital furrow distinct, 

 occipital ring moderately long (sag.), anterior border somewhat nar- 

 rower (sag.) than the preglabellar field medially ; marginal furrow 

 well impressed at the sides but shallow medially owing to a slight 

 rearward extension of the median portion of the border. Posterior 

 limbs appearing as wide (tr.) as the occipital ring in dorsal view, 

 actually somewhat longer because the distal, downtumed portion ap- 

 pears shortened. Distal portion of limb half as wide (tr.) as proxi- 

 mal portion. Distance from posterior end of palpebral lobe to posterior 

 margin appreciably less than length of palpebral lobe. Surface of 

 test finely pimctate. Length of figured cranidium 6 mm. 



