140 3fonograph of the Crustacea of the Cincinnati Group. 



These trilobites differ remarkably from Phacops in the structure of 

 their small eyes, which are seldom or never preserved, seeming to have 

 been of so delicate a nature as to fall out on the animal's death, leaving 

 a hole in the cephalic shield, where they were set, whence Dalman's 

 term "hiant," The head differs from Phacops in its blunt, lateral 

 angles, and the glabella being wider at base than in front, and in the 

 course of the eye line ; the body differs, in having thirteen instead of 

 eleven segments. 



Calymene senaria — (Conrad, 1841). 



Cephalic shield semi-circular or sublunate, regularly rounded in 

 front, or slightly projecting in front of the glabella, with a distinct 

 thoracic ring at the base ; posterior angles sub-acute or rounded ; gla- 

 bella separated from the cheeks by a deep, broad groove, wider behind, 

 or often of nearly equal width throughout, with three tubercles or 

 lobes on each side, the anterior one often obscure ; cheeks triangular ; 

 eyes truncate conical, situated a little outward from the inner edge of 

 the cheek ; facial suture terminating nearly in front of the eye ; thorax 

 with thirteen segments, those of the lateral lobes with a deep groove 

 extending from the base more than half way to the extremities ; py- 

 gidium small, with seven segments in the middle lobe, and five in each 

 lateral one, the latter with an impressed line or shallow groove the 

 whole length. 



It is found throughout the entire exposure of the Cincinnati Group 

 of blue limestone, at all elevations. It is sometimes rolled up and 

 sometimes extended. It varies in size from an eighth of an inch in 

 length to 2J inches. Good specimens are much more abundant than 

 those of any other species of trilobites, though in many places only 

 fragments are to be found. 



Some specimens are closely covered Avith tubercles, as those found 

 at the cutting for Columbia avenue, and at other places, from low 

 water-mark to 200 feet above, while they are smooth generally and ap- 

 pear never to have been tuberculous in the higher rocks, though this 

 is, according to Burmeister, the result only of having lost the tubercu- 

 lous covering. Different specimens present different proportions in 

 many parts of the body. The anterior part of the cephalic shield is 

 much longer and projects farther up in some specimens than in others, 

 while some are longer in proportion to their width than others, and 

 differ in other minor particulars. 



The Calymene callicephala (Green),'' described in 1832 in Green's Mo- 

 nograph, and sold as cast No. 2, was, in my judgment, a large speci- 



