NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 105 



doubtfully identified in Canada, by their straight ribs, not reaching the margin, and by the 

 absence of striae and granulations. The species is quite distinct, of course, from B. niaga- 

 rensis Hall (Pal. N. Y., II. 314, pi. Ixx. 3). 



Ill^nus Dalman. Bumastus Murchison. 



Illaenus (Bumastus) worthenanus W. and M. 



Plate III. figure 13. 



Head elevated, greatly inflated, the glabella closely imitating the superior portions of the 

 human cranium from the middle of the forehead over to the lamboidal suture. The lat- 

 eral lobes are separated from the glabella by shallow furrows which arise from pits on the 

 antero-lateral aspects, a little above the border, and arch upwards, backwards, and down- 

 wards nearly in a circular arc of 160°. The eyes are small, superficially elongate-elliptic, 

 bent over the most prominent portion of the lateral lobes in such position as to look exactly 

 at right angles to the vertical plane passing longitudinally through the middle of the ani- 

 mal. Their position is greatly posterior to, and a little below, the middle of the head. 

 The facial suture starts in the border, below the antero-lateral pit, rises with slight curva- 

 ture, at an angle of 45° to the anterior extremity of the eye, and thence descends rapidly 

 to the posterior border. The fixed cheek has thus a lunate form. The movable cheek is 

 limited in extent, its width being about equal to the length of the eye ; it extends back- 

 ward as far as the main lobe of the glabella. The occipital ring is extremely arched, giv- 

 ing the head a truncate appearance posteriorly. 



In young specimens the glabella is relatively more depressed, — the eye being halfway 

 up ; the eye itself is more prominent, with a sub-ocular furrow. 



The hypostome, in a young specimen, is nearly as convex as the glabella. It has the 

 form of a pentagon with the two sides opposite the base rounded and coalesced into a semi- 

 circle, — the curved side of the figure being anterior, the base posterior and transverse to 

 the animal's body. The two sides adjacent to the base are deeply isolated from the cen- 

 tral area by furrows which begin at the lateral angles of the pentagon and extend to the 

 base, in close proximity to the basal angles. The thorax is unknown. 



The pygidium is the figure included in the arcs of two circles, the anterior of which is of 

 greater radius, and embraces about G0° — the posterior embracing about 160.° A belt around 

 the margin is somewhat depressed, but no trace of trilobation is apparent. 



No portion of the test is preserved in any of our numerous sjiecimens ; and the casts do 

 not exhibit any traces of superficial ornamentation. The head of the largest individual is 

 1.9 inches long, 2.2 broad, and 1.4 high. The narrowest part of the glabella, which is 

 between the eyes, is 1.15 inches across, and the widest part — the front — 1.45 inches across, 

 the width next the collar being very nearly the same. 



This striking species might, on casual comparison, be referred to Bumastus barriensis Mur- 

 chison, but on extended consideration we have been induced to regard it as decidedly dis- 

 tinct from the types of that species. On the side view of the head the anterior portion is 

 more depressed; the eye is below the middle instead of above it; it is horizontal instead of 

 oblique ; and the facial suture, instead of ascending almost vertically, rises at an angle of 

 45°. On the view from above, the head is relatively longer and narrower, and the furrows 

 isolating the glabella much more distinct. Moreover, the pygidium of B. barriensis (aut 

 rectius barrensis) is more than a semicircle, — in our species considerably less ; it is also much 

 more depressed, and its border is flattened. 



MEMOIRS HOST. SOC. NAT. HIST. Vol. I. 27 



