64 PROCEEDINGS OF THE ACADEMY OF 



52, and to its own breadth of 79 to 107, being very slightly wider near the 

 middle than in front, and narrowing posteriorly, with gently convex lateral 

 margins, from behind the middle to the pygidium. Axial lobe depressed, nar- 

 row, or only about two-thirds the breadth of each lateral lobe at its anterior 

 end, and narrowing regularly with straight sides posteriorly ; segments thir- 

 teen, nearly or quite straight, and each with a small node or prominence at 

 each end."* Lateral lobes depressed or nearly flat ; pleurae almost transverse 

 or arching slightly backward, to near the extremities, which are abruptly 

 pointed ; each with a well defined furrow, which commences small near the 

 anterior inner end, and widens and deepens for about half-way out, and then 

 narrows and becomes more shallow, so as to die out before reaching the late- 

 ral extremities. 



Pygidium subsemicircular, being rounded posteriorly, with a narrow, 

 slightly flattened border, and somewhat rounded anterior lateral extremities ; 

 length bearing to that of the thorax the proportions of 30 to 79. and to that 

 of the head of 30 to 52, with a breadth of not quite two-thirds of that of the 

 head ; axial lobe more than two-thirds the length, narrow, depressed, and 

 showing more or less distinctly about five segments ; lateral lobes much de- 

 pressed, nearly twice as wide at the anterior end as the middle one, each with 

 about three segments, which curve a little backward and become obsolete be- 

 fore passing upon the narrow smooth border ; segments each provided with a 

 comparatively large longitudinal furrow, corresponding to those on the 

 pleura. 



Entire surface apparently smooth, excepting fine radiating strias on the an- 

 terior aud lateral portions of the cephalic shield that are scarcely visible with- 

 out the aid of a magnifier. 



Whole length, 1-60 inch; breadth of thorax, 1-07 inch; do. of cephalic 

 shield (somewhat flattened by pressure), about 1-12 inch; length of thorax, 

 70 inch ; do. of pygidium, 0-30 inch ; breadth of do., 0-60 inch. 



Of this fine Trilobite three entire specimens and a part of another were ob- 

 tained. They are, however, all merely sharply defined natural casts, formed 

 by the deposition of a crust of arragonite in the original moulds left by the 

 fossil in some kind of a matrix. The specimens were evidently somewhat 

 flattened by pressure before or at the time they left their impressions in the 

 rock. This compression has obscured the lateral furrows of the glabella, but 

 most of the other characters of the upper side of the fossil are clearly seen, 

 even to the facial sutures, and the faintly marked radiating striae seen around 

 the front and lateral margins of the cheeks. 



The genus Conocephalites (or more properly Conocori/phe, for a strict applica- 

 tion of the rules of priority would, I should think, require that the latter name 

 should be adopted for the genus to which they were both applied) is so nearly 

 allied to Olenus that it may not be always easy to distinguish the two types 

 without seeing the hypostoma, and hence it is possible that the form under 

 consideration may be more properly an Olenus. As it has more the regular 

 oval outline of the former, and less pointed and produced pleura than the lat- 

 ter, while it shows clearly the fine radiating striae around the anterior and 

 lateral margins of the head so often seen in Conocori/phe, it more probably be- 

 longs to that genus. It is worthy of note, however, that all of the specimens 

 seem to be much more depressed or flattened than any of the species yet de- 

 scribed of that genus, while only one of them shows any traces of the slender 

 ridge usually seen passing from the anterior end of each eye to the front ex- 

 tremity of the glabella, and in this one the ridge is so faintly marked as to 

 leave doubts whether or not it is natural. 



Locality and position. Antelope Springs, Dryont Mountains, Nevada. Lower 

 Silurian, and probably, judging from the known position of the genus Conoce- 

 phalites, in the rocks of this country and Europe, from the Primordial zone. 



* In some specimens these nodes seem to be wanting, while in others they do not exist 

 on all of the segments. 



[April, 



