80 



umbonial cavity there is a strong oval or subcircular callosity about 1 line 

 in width in shells which are 3 or 4 lines in length. Above this the um- 

 bonial cavity is deeply excavated and the dental plates scarcely at all 

 developed. In the interior of the dorsal valve a lower obtuse ridge runs 

 along the median line from the foramen for half the length of the shell 

 forward, when it becomes obsolete. No divaricator process can be seen 

 in the foramen of any specimen observed, although this part appears to be 

 perfectly preserved in many of the detached valves examined. 



Surface covered with fine bifurcating striae from six to eight in the width 

 of one line. 



Width from 3 to 6 lines ; length about - or less than the width. 



As before mentioned, this species is very closely allied to 0. perveta, 

 the geological position of which appears to be about the Chazy, Black 

 River, and the base of the Trenton limestones. The only differences that 

 can be made out from a comparison with specimens from Tennessee and 

 the figures given by Hall in the Palaeontology of New York, are, that in 

 0. perveta the dorsal valve is more convex than it is in 0. electra, and the 

 beak of the ventral valve not so depressed, while at the same time it is 

 more extended. At present I have no means of comparing the interiors 

 of the two species. When such a comparison can be made, should no 

 greater differences be disclosed than are afforded by the external charac- 

 ters, I would be disposed to unite the two under one name. The specimens 

 of 0. perveta from Tennessee vary greatly in form and convexity, and yet, 

 in all, the dorsal valve is more convex that it is in 0. JSlectra. The 

 European species 0. parva is also most closely allied to 0. Mectra, only 

 differing therefrom, so far as can be determined by comparison with figures, 

 in having the beaks more strongly incurved. Its geological position must 

 be very nearly the same. 



These species belong to a group of finely striated Orthides, which made 

 its appearance in the beginning of the Silurian period, and flourished in 

 all the after geological ages up to the Carboniferous. 



Locality and Formation. Point LeVis. The specimens from which 

 the description was drawn up are silicified shells, procured in the upper 

 part of limestone No. 2. But in the white, grey, and yellowish masses 

 which hold the greater number of the species of trilobites, a species 

 resembling this occurs rarely, but not sufficiently well preserved to enable 

 us to say whether or not it is the same. 



Collectors. Sir W. E. Logan, J. Richardson, R. Bell. 



