THE NATURAL HISTORY SURVEY. 215 



Family 2. AS APHID AE Emmrich. 



"Cephalon and pygidium well developed; glabella often 

 obscurely limited. Free cheeks usually separate. Facial sutures 

 extending forwards from the posterior edge of the cephalon 

 within the genal angles, and cutting the lateral or anterior 

 margins, occasionally uniting in front of the glabella. Eyes- 

 usually present, smooth, w^ell developed, sometimes of very 

 considerable size, even occupying the entire surface of the 

 free cheeks. Thorax generally composed of eight or ten seg- 

 ments, but varying from five to ten. Pygidium large, often 

 with wide doublure." Beecher, Zittel-Eastman Text Book of 

 Paleontology, p. 629. 



Genus 2. ILLAENUS Dalman, 1826. 



Complete body sub-elliptical in outline, the trilobation 

 often nearly obsolete. Cephalon large, strongly convex, 

 smooth except for the slightly impressed dorsal furrows and 

 the eyes. Eyes large, situated laterally and posterior to the 

 mid-length of the head, their surfaces sometimes protruding 

 beyond the' lateral margins of the head. Anterior limb of the 

 facial suture describing a gentle, slightly sigmoidal curve from 

 the anterior margin of the cephalon to the eye j , the posterior 

 limb curving rather abruptly laterally from the posterior ex- 

 tremity of the eye to the post-cephalic margin. Free cheeks 

 usually triangular. Thorax transversely convex, with nine or 

 ten broad segments. Pygidium similar to the riead in form 

 and size, but usually less convex, the axis short and inconspicu- 

 ous, sometimes obsolete. 



Illaenus insignis Hall, pi. xvii, figs. 1-5. 



Description. Cephalon large, parabolic in outline, strongly 

 convex, the most prominent point on the median line between 

 the centers of the eyes; anterior and lateral margins slightly 

 produced to form a narrow, thin, projecting, lip-like border. 

 Dorsal furrows moderately impressed, extending from the 

 posterior margin of the head nearly to the anterior border 

 where they terminate in shallow, pit-like depressions. 'Eyes- 

 large, sub-crescentiform, convex both vertically and horizon- 

 tally, from two and a half to three times as long as high, situated 

 a little obliquely, the posterior extremities being a little higher 

 than the anterior, with the center of the eye a little below the 

 center of the total height of the cephalon and a little nearer to 

 the posterior than to the lateral border. Eye-lobes sloping. 



