62 PROCEEDINGS OP THE ACADEMY OP 



size. It is much more nearly allied to a form now before me in masses of 

 chert from the west side of Lake Pepin, in Minnesota, found in beds of about 

 the age of the Calciferous Sand rock of the New York series. The latter, 

 however, attains a much larger size, some of the specimens being an inch in 

 diameter, with about six volutions. The Minnesota form is also more sharply 

 angular on the periphery, and has the upper side of the volutions distinctly 

 more concave, and the spire more depressed.* 

 Locality and position, same as last. 



Paradoxides ? Nevadensis, Meek. 



The only specimen of this Trilobite obtained, consists of a natural cast, 

 formed by a moderately thick crust of Arragonite, deposited in a mould or 

 impression of a part of the thorax and the pygidium, with the free borders of 

 the latter broken away. Its rather large size, much depressed form, spinife- 

 rous pleura, and general physiognomy, as far as seen, at once recall to the 

 mind the well known genus Paradoxides. A closer inspection, however, shows 

 its pygidium to be proportionally larger than we see in the known species of 

 that genus, with possibly the exception of P. Forchhameri, of Angelin. 



Of the thorax, eight of the posterior segments are preserved. These show 

 the axial lobe to be much depressed, and about as wide as the lateral ones, 

 exclusive of the free recurved points of the pleura. The segments of the 

 axial lobe are defined by a broad rounded furrow or depression across the an- 

 terior side of each, and have much the general appearance of those of some 

 species of Paradoxides, being a little thickened, squarely truncated, and 

 slightly curved forward at the ends. But they differ in showing distinct re- 

 mains of a mesial spine or tubercle on each, and in having an obscure, oblique 

 furrow or depression on each side, passing outward and backward from the 

 broad anterior transverse furrow to the posterior lateral angles, so as partly to 

 isolate the slightly thickened and truncated extremities of each. The lateral 

 lobes are nearly flat, and composed of pleurae that extend straight outward at 

 right angles to the axis, to their free extremities, which are abruptly con- 

 tracted (almost entirely on the posterior side) into slender, rounded, very 

 sharp spines, which curve backward and outward. Each of the pleura? is also 

 provided with a broad, rather deep, flattened furrow, .which commences near 

 the inner end, and extends straight outward for some distance, wiih parallel 

 sides, but gradually tapers, mainly on the anterior side, to a lanceolate point, 

 before reaching the free extremities. These furrows have not the obliquity 

 usually seen in those of Paradoxides, but run parallel to the direction of the 

 pleurae, so as to leave slender straight ridges, of equal size, along the anterior 

 and posterior margin of each rib. 



The pygidium, exclusive of the portions of the free border broken away, has 

 a nearly semicircular outline, being about twice as wide as long, while it is as 

 much flattened as the thorax. The part remaining equals in length the five 

 thoracic segments next in advance of it. Its mesial lobe is much depressed, 

 and about three-fourths as wide, anteriorly, as the breadth of that of the tho- 

 rax at its widest part seen. Posteriorly it tapers very little, and extends 

 nearly the entire length of the pygidium, as seen with the free border 

 broken away. It is evident, however, that the flattened border projected more 

 or less behind its termination. It shows distinctly five segments, with indica- 

 tions of about two others at the posterior end. The lateral lobes have each 

 three segments, the anterior one being extended out nearly parallel to those of 

 the thorax, while the others are directed more obliquely backward, and rap- 

 idly widen outward. Like the pleura?, they have each a broad flattened fur- 

 row, that of the anterior one being nearly parallel to those of the pleura, 



* This Minnesota shell probably belongs to an undescribed species, as it has more volu- 

 tions than the form described by Dr. Owen, under the name Stroparollus {Euomphalus) Min- 

 nesotensis. (See his large Geological Report of 1852, p. 581, pi. 11, rig. 12 and 13.) If so, it 

 might be called Euomphalus Pepimnsis. 



[April, 



