116 E. J. CHAPMAN 



Order TKII.OBITA. 



Sub-Ordcr I.— PUSILLIFORMES.— Small, aberrant trilobites, with typically two thoiaoic segments, 

 and head-sliield and pygidinm of nea, '3' ecjjiial size. 



Fam. 1. Agnostidw. — Typical <^enus, Agnostus, Cambrian, Lower Silurian. 



SnlvOrder IT.— LATIFOEiLFS :— More or loss broad, often Iar<re, trilobitcs, with head-shield, thorax 

 and pyj^idium of nearly equal dimensions. Bod^'-seifments t3-])ically 8 to 10 in number. 

 Surface of shell commonly marked with wavy marginal lines. 



§ 1. Levati. — Broad-formed or oval trilobites, with essontially smooth shell, and inconspicuous or 

 feebly elevated glabella. 



Fam. 2. lllatiicLv. — Pleura; unfurrowcd. Pygidium with smooth sides and smooth or feebly fiii-i-ow- 

 ed axis. T^-p. gen. : i/Zfrnus, Up. Cam., Sil. ; Bumastus,^\\.; i\7ta/5, Lr. Sil. ; S^'y/nfl, Lr. Sil. 



Fam. 3. Asaphida: — Pleura) fun-owed. Pygidium with smooth sides and smooth or feebly furrowed 

 axis. Typ. (Jen.: Ilhvtmrus, Cam.; Niohe, Lt. Sil. ; AsapJan', Lr. Sil. ; Psilocephalus, Cam.; 

 Barrandia, Lr. Sil. ; Bnthyiinllus, Lr. Sil.; Batlujurus, Cam., Lr. Sil.; Megalaspis, Li-. Sil. 



Note. — The two families of this section, although widely separated in the classification of 

 Barrande, arc very closely allied, and are commonly united in more recent systems, principally from 

 the fact that certain genera — lllsenurus, Niobe and Stygina, more especially — appear to form an 

 intermediate or transition group. But the tj-pical ''epresentatives of each famil}' are clearly charac- 

 terised, and connecting points of this kind are ab. ndantly traceable in the case of other families 

 universally regarded as distinct. In the present classification, indeed, the collujation of the families 

 is based, as far as this can be carried out, on the existence of these connecting or transitionary forms. 



As regards Family 3, I have separated from the Asajjliidfc proper the forms with horned head- 

 shield and strongly furrowed ]iygidiuni, and have placed these in a distinct family under the generic 

 name of Basilicus, already applied to them, as a sub-genus of Asaphus, by Salter. This is warranted 

 by the very marked contrast presented by these strongly furrowed, lobate and conspicuously horned 

 forms, to the smooth IllffinidiC and Asaphida-. The horned genus, Megalaspis, forms the connecting 

 link. 



§ 2. SuLCATi. — Broad-formed or oval trilobites with horned head-angles, pointed pleurir, and large, 

 strongly-furrowed pygidium. 



Fam. 4. Basilicidce — Glabella with short side-furrows, cr smooth. Typ. gen. : Basilicus (j= Asaphva 

 in part), Lr. Sil.; Oxi/gia, Lr. Sil.; Jfomalotopeon. Lr. Sil. 



Fam. 5. Dikeloceplialidm. — Glabella strongly- developed, ivith transverse or side furrows. Number 

 of body-segments unknown. Typ. gen. : Dikelocephalus, Cam.; D or ijptjge, Cam. 



Note. — Dikelocephalus, as a Cambrian type, is commonly placed with Parodoxides and other 

 Cambrian forms under the family of the OlenidiB. But fiom these it is distinctly separated by its 

 large pygidium. This latter character, together wilh its horned head-shield and the meeting of its 

 facial sutures in a spado like point above the glabella, connects it far more closely with the horned 

 and furrowed Asaphidic or Basilicida-, in many of which, as well as in many species of the Asaphidio 

 proper, the course of the sutural outline is the same. '^'. '"eater number of Iwdy-segments may 



