OLENEI.LUS AND OTHER GENERA OF MESONACID^ 275 



Cephalacanthus Lapworth (in part), 1891. Geological Magazine, Dec. 3, 

 Vol. 8, p. 531. (Gives reasons for proposing the genus. The reference 

 to the original place of publication of Cephalacanthus is given as " Geo!. 

 Mag., 1888, p. 641 " it should be " Tenth Ann. Rept. U. S. Geol. Survey, 

 by Chas. D. Walcott, 1891, p. 641.") 



Holmia Peach and Horne (in part), 1892, Quart. Journ. Geol. Soc. Lon- 

 don, Vol. 48, p. 236. (As defined this genus includes forms now re- 

 ferred to both Holmia and Callavia.) 



Holmia (Olenellus) Peach (in part), 1894, Idem, Vol. 50, pp. 671-674. 

 (As discussed in these pages this genus includes forms now referred 

 to both Holmia and Callavia.) 



Callavia Matthew, 1897, American Geologist, Vol. 19, p. 397, footnote. 

 (Generic name proposed to include "Olenellus broggeri" Walcott 

 and " Olenellus callavii" Lapworth on account of the glabella differing 

 from that of " Olenellus (Holmia) kjerulfi.") 



Holmia Moberg (in part), 1899, Geol. Foren. i Stockholm Forhandl, Bd. 

 21, Hafte 4, pp. 314 and 318. (As characterized the genus includes two 

 species now placed under Callavia.) 



Dorsal shield broad ovate, moderately convex ; cephalon broad, 

 semicircular ; marginal rim broad and continued into genal spines ; 

 posterior margin with a strong, short intergenal spine just within 

 the genal angle and the rudimentary facial suture. 



Facial sutures rudimentary or in a condition of synthesis back of 

 the eye, but not observed in front of the eye. Eye lobes narrow, 

 elongate-crescentiform. Glabella clavate-elongate, with the large 

 anterior lobe contracting toward the front. The three posterior 

 lobes are not strongly defined, the occipital ring has a strong median 

 spine extending back over the thorax. 



Hypostoma convex, broad in front, narrowing to the broadly 

 rounded, smooth posterior margin ; crossed within the narrow pos- 

 terior margin by a sulcus subparallel to the margin, also a flattened 

 ridge anterior to which a strong groove is outlined on each side ; 

 antenna! furrows, x x, fig. 2, pi. 2y, gently arched inward on the 

 lateral margins. 



Thorax with fifteen to eighteen segments. Axial lobe convex, 

 with an elongate node or spine at the center. Pleural lobes broad 

 and passing gradually into the broad, curved extensions of each seg- 

 ment ; pleural furrows extending from the side of the axial lobe out 

 to the beginning of the curved terminations of the pleurae. 



Pygidium small, transverse, and with a transverse groove near the 

 anterior margin; lateral lobes^not developed. 



Surface minutely granular and with irregular network of very 

 narrow, irregular ariastomosing ridges. 



Genotype. — Olenellus (Holmia) broggeri Walcott. 



