NO. 3 CAMBRIAN TRILOBITES 20I 



The pygidia may be arranged in two groups : First, the C. iowen- 

 sis group, in which the postero-lateral margin of the pygidium 

 extends backward on each side from a broad base into a sharp 

 narrow spine. Second, the C. texanns group, in which the postero- 

 lateral spines are long, slender, and attached to the side of the pleural 

 lobe above the margin. The latter appears to be the oldest form, 

 as it occurs with C. augnsta and C. liliana (pi. 29) of the upper beds 

 of the Lower Cambrian, and also with the Middle Cambrian species 

 of China, C. convexus^ One of the Chinese species, C. damia, has 

 an associated pygidium " that is similar in appearance to the pygidium 

 of C texanus. 



The surface of the C. iowensis group of species is smooth to the 

 unaided eye, but slightly roughened by fine pitting when seen with 

 a strong lens. The C. texanus group of species all have a more or 

 less decided granulation. The test of all known species of the genus 

 in which it is preserved is pitted and apparently punctate. 



The various species now referred to Crepicephalus may be grouped 

 as follows : 



Crepicephalus iowensis group : Test nearly smooth, probably 

 punctate, frontal furrow of cranidium narrow and simple ; pygidium 

 transverse with border extended into broad-based sharp postero- 

 lateral spines. Species: C. iowensis (pi. 29), C. camiro (pi. 32), 

 C. convexus (China), C. coosensis (pi. 32), C. dis (pi. 32), C. mag- 

 nus (China), C. undt. spp. (pi. 32, figs. 4 and 4a). 



Crepicephalus texanus group : Test granulated, probably punc- 

 tate ; frontal furrow of cranidium usually broad and strong, with 

 three marked pits in front of the glabella; pygidium slightly trans- 

 verse with a narrow, long spine coming out of the pleural lobe on 

 each postero-lateral side. Species: C. texanus (pi. 30), C. comus 

 (pi. 31), C. coria (pi. 33), C. damia (China), C. texanus danace 

 (pi. 29), C. texanus elongatus (pi. 29), C. thoosa (pi. 31), C. tri- 

 punctatus (pi. 33), C. tripunctatus magnispinus (pi. 33), C. tumidus 



(pl.3i)- 



The species C. tripunctatus and variety magnispinus differ from 

 all other known species of the genus in having a strong occipital 

 spine. 



Genotype. — Dikeloccphalus ? iowensis Owen.* 



' Research in China, Carnegie Institution of Washington, Vol. 3, 1913, pp. 

 140-142, pi. 13, fig. 16&. 

 ^ Idem, pi. 13, fig. 14&. 

 ' Rept. Geol. Surv. Wis., Iowa, Minn., 1852, p. 575, pi. i, fig. 4; pi. lA, fig. 13. 



