106 WINCHELL AND MARCY ON FOSSILS FROM THE 



We hesitate to insist upon these distinctions after the foreign species has been recognized 

 in this group, in New York, by Prof. Hall. But without expressing any opinion of that 

 identification we may say that the head of our species is much narrower relatively than 

 that figured by Hall, and the glabella has a greater development posterior to the line of 

 the eyes. Some of the pygidia of Hall's species greatly resemble ours,— e. g. Plate 66, fig. 

 2, Pal. N. Y, Vol. II. This species should also be compared with lllmms imperator Hall 

 (Wis. Geol. Rep. 1861, p. 49,) from the Racine limestone. 



Acidaspis Murchison. 



Aeidaspis Ida W. and M. 



Plate III. figure 13. 



Head more than twice as broad as long, transversely subelliptic in outline, very tumid 

 Glabella consisting of a median body and three lateral lobes on each side. The median 

 body is twice as long as broad, and narrows somewhat in front of the anterior lateral lobes 

 It rises in a steep arch anteriorly and posteriorly, being somewhat flattened in the middle' 

 A slight protuberance is generally visible in the middle between the anterior lateral lobes' 

 The median body is separated from the lateral lobes by shallow "false furrows," uniting the 

 extremities of the lateral furrows which sink into deep pits between the lateral lobes 

 The dorsal furrow is extremely shallow and indistinct, so that the lateral lobes are nearly 

 confluent with the fixed cheeks. The middle pair of these lobes are about twice the size of 

 the others; and the anterior are smaller than the posterior. Occipital furrow deep its late- 

 ral bifurcations sinking into deep pits and isolating the posterior lateral lobes. Occipital 

 ring with a pair of slender, slightly diverging spines, gently curved downward toward the 

 extremity, and having a length of about one inch. Between the bases of the spines is a 

 small tubercle which, in good specimens, is seen to become a short erect spine. 



The fixed cheek is convex, somewhat pyriform in outline. 



The ocular fillet is well defined along the anterolateral border of the fixed cheek as far 

 as the eye, which is situated upon a slender, diverging peduncle, about three tenths of an 

 inch high, arising opposite the middle of the cheek. 



The movable cheek is broadly crescentiform, of medium size, about as broad as the main 

 lobe of the glabella, projecting forward a little further than the frontal border. Its plane 

 in the broadest part makes, with a vertical plane through the axis of the animal, an inferior 

 angle of about 35°, but anteriorly to this the border is turned upwards, giving a concave 

 upper surface to that part of the cheek. The anterior border is thickened and^ornamented 

 with numerous small tubercles arranged at regular distances ; the posterior angle is drawn 

 out, and terminates in a slender spine about one inch in length and standing at an angle of 

 55° with the axis of the animal. The inner side of the base of the spine °is continued so 

 far as to deeply indent the posterior border. 



The entire surface, except the occipital ring, is covered with granules of unequal size 



Nothing is known of the other parts of the animal, except that a fragment, probably 

 the border of the pygidium, is armed with short, stout spines. ' 



Transverse diameter of cephalic shield (in a straight line), 1.65 inches ; longitudinal diame- 

 ter, .87 ; width of median body of the glabella, .46 ; width of fixed cheek, .25 ; width of 

 movable cheek, .31 ; middle of glabella elevated above outer border of movable cheeks .36 

 inch. 



