SILURIAN AGE. 



ARTICULATA. 



Class CRUSTACEA. 



Order] Trilobita. 



Family PARADOXIDJE. 



Head well developed, sometimes very large ; facial sutm^es generally 

 subparallel, especially the anterior portion ; ribs each provided with a 

 longitudinal furrow. Thorax large, consisting of from twelve to twenty 

 segments. Pygidium very small, and always with few segments. 



This family includes the genera Paradoxides, Olenus, Olenellus, Peltura, Sao, 

 Hydrocephalus, Triarthrus, Agraidos, Ellipsocephalus, Gonocoryphe,^ and probably 

 Ptycliaspis, Chariocephalus, CrepicepJialus and Menocephalus. It embraces a con- 

 siderable number of species, some of which attain a very large size. They are 

 generally remarkable for the great development of the thorax, compared with the 

 small size of the pygidium. The whole family, with the exception of Triarthrus, 

 seems to be mainly, if not entirely, confined to the Primordial or oldest group of 

 fossiHferous rocks. 



Genus AGEAULOS, Corda. 



Synon. — Arion, Bareande, Note Prelim. 1846, 12 (not Ferussac, 1819). 



Herse, CoKDA, Prodr. 1847 (not Oken, 1815, nor Lesson, 1837). 



Agraulos, Corda, ib. 1847. 



Arionides (Baekande, MSS.), Bkonn, Index PaL, 1848, 103. 



Arionellus, Baeeande, Syst. SU. Boh. 1852. 



Crepicephalus? (part), Owen, Report GeoL Survey, Wisconsin, Iowa and Minnesota, 1852, p. 576. 

 Etymol. — Aypai;Xof, daughter of the first King of Athens. 

 Type. — Arimi ceticepltalus, Bareande. 



Entire animal more or less elongate-ovate, distinctly trilobate. Head forming 

 more than a semicircle, nearly straight behind; glabella conoid-subovate, provided, 

 in young examples, with three or four lateral furrows, which are usually nearly or 

 quite obsolete in the adult, margined in front by a more or less developed border 

 connecting with the cheeks on each side. Facial sutures widely separated, ex- 

 tending and converging forward from the eyes so as to intersect the anterior 

 margin within a point where a line would strike it if drawn through each eye, 



^ Gonocoryphe, Corda, 1847 = Gonocei:)halus, Zenker, 1833 (not Thunberg, 1812) = Conocepha- 

 lites, Barrande, 1852. 



