126 Monograph of the Cnidacca of the Cwcinnati Group. 



Trilobites, in point of the funtlamental numbers of their thoracic rings 

 are not constructed according to the law Avhich we have discovered 

 to obtain among all Crustacea of the present world. 



The earlier types of creation seem to present the various peculiarities 

 of several groups passing into one another, resulting in forms, which 

 exhibit in association, although incompletely the peculiarities now found 

 detached and characteristic of very distinct groups. In proportion to 

 the geological age of the extinct species, the running of the various 

 typical forms into one structure is more marked, and therefore the 

 peculiar and organic individuality and distinctiveness less obvious. 



Prof. McCoy divided the family TrUohitadce into five sub-families : 

 lat, Agnostince 2d, Harpedince ; od, Ogygince ; 4ih, Paridoxincc ; 5th, 

 Asaphince. 



Third Subfamily Ogygince, 



Body flat, broad, oval; thorax as long as the head, four to eight 

 thoracic segments ; pleurte flat, sickle-shaped, horizontal, having a 

 pleural furrow not reaching the margin, ends not produced into spines ; 

 jio facets, pygidiurn nearly as large as the head ; eyes small or absent. 



The Ogygince include the flat-sided trilobites not entering into the 

 group of Faridoxincc ; unlike them, however, the body is very short 

 and wide, and the pygidium, instead of being diminutive, is nearly as 

 large as the head, giving a broad oval form to the body ; the thoracic 

 segments are only from four to eight in number, never facetted, and 

 their exti-emities are never prolonged into spines. 



This subfamily includes the geaera Trimieleus Lichas, etc. 



Genus Tmmckus — (Lhwyd 1698). 



Cephalic shield semi-circular, the limb broad and flat, with several 

 rows of impressed, cup-like puncta on both surfaces ; spines of the ex- 

 ternal angles very long ; glabella pyriform, without lateral furrows ; 

 cheeks spherical triangles, without eyes or eye lines ; generally a small 

 spine on the neck furrows ; tliorax of six joints, lateral lobes twice the 

 Avidth of the axis, flat, straight, with parallel sides, slightly deflected 

 at their free ends, pleural groove broad, shallow, not reaching the mar- 

 gin ; pygidium triangular, with a deflected margin, axis with about 

 seven segmental furrows, sides flat, with about six segmental divisions, 

 each subdivided by a faint line toward the end. 



Trimieleus Concentricus, — (Eaton, 1832). 

 Cephalic shield nearly semi-circular, the posterior angles produced 



