94 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



Phacops rana. Body elongate ; length three times the width ; sides 

 nearly straight ; head almost perfectly semicircular, except that the po- 

 sterior angles project beyond the line. Glabella very gibbous, wider than 

 long, with faint marks of the lateral lobes. At the basal angles of the 

 glabella are rounded or ovoid tubercles, and, below the centre, a trans- 

 verse elevation, with a few small papillae like those of the surface : the 

 occipital furrow is narrow, and the occipital ring wide and strong ; cheeks 

 spreading and rounded at their posterior extremities ; the border is every 

 where narrow and even. The eyes are of moderate size and neatly placed, 

 rising nearly as high as the plane of the top of the glabella in well pre- 

 served specimens. In young specimens, there are five and rarely six lenses 

 in the vertical rows ; while in older specimens there are usually four lenses, 

 the thickening of the palpebral lobe having obscured them. In a well- 

 preserved specimen of medium size, the eyes ( beginning on the anterior 

 side) have one vertical row with four lenses, nine rows with five each, two 

 rows with four each, two rows with three each, two rows with two each, 

 and one lens in the posterior angle, giving altogether sixty-eight lenses 

 in each eye. 



The length of the thorax is equal to the width at the posterior end, which 

 is about a seventh or eighth narrower than the anterior end. The axis is 

 regularly rounded and moderately elevated ; the lateral lobes flat for 

 half their width, and somewhat abruptly bending at the sides. The pygi- 

 dium is more than twice as wide as long ; the axis marked by eight or 

 nine, and rarely by ten rings. Seven or eight ribs may be counted in the 

 lateral lobes of young specimens, the posterior ones becoming obsolete in 

 older individuals. 



This species occurs in large numbers, and specimens have been found 

 from the size of half an inch to nearly three 'inches in length ; and from 

 measurements of separate I heads and bodies, we infer that individuals have 

 been three inches and a half long. It agrees more nearly with the Phacops 

 fecundus of Barrande, than with any other species known to me ; and it 

 would not be difficult to establish two or three varieties among our speci- 

 mens. 



Geological formation and locality. In the limestone of the Upper Hel- 

 derberg group : in the Helderberg mountains, Schoharie, and throughout 

 New-York ; and in the Hamilton group everywhere, but more particularly 

 in Central and Western New- York, as at Seneca and Cayuga lakes, Moscow. 

 Geneseo, and other placea. 



A specimen from Iowa, which T suppose to be from the same geological 

 position, presents no important points of difference. 



[Septeciber, 



