98 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



rows is always present and is usually united across the glabella ; a 

 second pair is usually short, but sometimes deep and extending en- 

 tirely across the glabella. Fixed cheeks wide, with very large pal- 

 pebral lobes in most of the species. In the type species they are more 

 than three-fourths as long as the glabella. Most of the species of 

 Irvingella have a narrow transverse frontal border but no rim. 



Free cheeks very narrow, in most species being little more than 

 a band passing around the eyes and filling in the notch anterior to 

 them and meeting the very short, and usually blunt, postero-lateral 

 limbs. So far as known the free cheeks have long genal spines. 



Thorax unknown except for isolated segments, in which the eleva- 

 tion of the axis usually agrees with that of the transverse section of 

 the head. 



The pygidium has a high, wide and long axis and flat sides. Several 

 rings are usually present in the axis. The sides may or may not have 

 pleurae indicated. Border often turned up into a wire-like rim. 



Derivation of name. — -Proposed in honor of the late Professor 

 J. D. Irving. 



Genotype. — Irvingella major Ulrich and Resser. 



Range. — Upper Cambrian and Ozarkian : Appalachians and Mis- 

 sissippi Valley, Rocky Mountains, and Nova Zemlya. 



Observations. — Irvingella resembles Chariocephalus very closely ; 

 in fact the two genera approach so nearly that it is almost necessary 

 to draw an arbitrary line separating them. Irvingella is distinguished 

 by its larger eyes, usual presence of glabellar furrows — sometimes, 

 however, not any more visible than in Chariocephalus — larger fixed 

 and smaller free cheeks and the better definition of the axis of the 

 pygidia. 



More than 25 species have been determined in about one-half of 

 the collections in the National Museum from the Upper Cambrian and 

 Ozarkian, in which the genus is widely distributed. 



IRVINGELLA MAJOR Ulrich and Resser 



Plate 15, figs. 26-29 



Irvingella major Ulrich and Resser in Walcott, 1924, Smithsonian Misc. 

 Coll., Vol. 75, No. 2, p. 58, pi. ID, fig. 3. 



The illustrations, particularly figure 26 which has been carefully 

 restored, show the characters of this species. All the specimens, 

 about 20 in number, both complete cranidia and fragments, are con- 

 tained in a single hand specimen. 



Formation and locality. — Upper Cambrian: (Son) Micaceous shale 

 member of the Franconia formation, Ableman, Wisconsin. 



