INVERTEBRATES. 497 



ARTICULATA. 



Genus ASAPHUS, Brongniart. 



ASAPHXJS (ISOTELUS) VIGILANS, M. and TV. 



PI. 23, Fig. 6. 

 Aiaphus (Isotelus) rigilans, Heek and Worthen, 1870. Proceed. Acad. Nat. Sci., Pliilad., p. 53. 



Body small, elliptic in general form, and moderately convex. Head 

 rather more than half as long as wide, approaching a sub-crescentic 

 outline, with the posterior lateral angles abruptly rounded or subangn- 

 lar; anterior margin apparently somewhat narrowly rounded ; posterior 

 outline broadly and distinctly concave, but rather straight along the 

 middle, without any traces of marginal or occipital furrows. Glabella 

 not rising above the general convexity of the head, and entirely unde- 

 fined by any traces of dorsal furrows. Eyes situated about their own 

 antero-posterior diameter in advance of the posterior margin, and appa- 

 rently about half way between the Tatter and the front, rather widely 

 separated from each other, and very prominent, nearly round, andtrun- 

 cato-sub-conie in form ; visual surface elevated almost entirely above 

 the general convexity, and curved around so as to form about three- 

 fourths of a circle, presenting a smooth surface ; palpebral lobes as ele- 

 vated as the eyes, and much contracted, or merely connected with the 

 glabella on the inner side by a narrow neck. Facial sutures extending 

 obliquely outward and backward from the eyes behind so as to intersect 

 the posterior margin about half way between a line drawn longitudin- 

 ally through the middle of each eye, and posterior lateral margins of 

 the cheeks; and in front, at first curving slightly outward a little in 

 advance of each eye, beyond which point they converge forward so as 

 apparently to intersect the front margin in such a manner as to leave a 

 rather narrow anterior edge to the glabella.* 



Thorax longer than the head or pygidium, as measured over the curve 

 of a rolled-up specimen, showing scarcely any traces of trilobation, and 

 composed of eight segments. Mesial lobe, as indicated by very faint 

 impressions on each side of the body segment, very wide and depressed, 

 with segments nearly flat. Lateral lobes very narrow, sloping off regu- 

 larly from the mesial one on each side ; pleurae without furrows, and 



* A3 the specimen is imperfect here, it is possible these sutures may not reach the anterior margin 

 in front. 



