116 E. J. CHAPMAN 



Order TKII.OBITA. 



Sub-Order J.— PUSILLIFOEMES.— Small, aberrant trilobites, with typicallj- two thoracic sogincntM, 

 and head-shield and pygidium of nearly equal size. 



Fam. 1. Agnostidœ. — Typical genus, Agnostus, Cambrian, Lower Silurian. 



Sub-OrderlL— LATIFOEMES:— More or loss broad, often large, trilobites, with head-shield, thorax 

 and pygidium of nearly equal dimensions. Body-segments typically 8 to 10 in numbei-. 

 Surfjice of shell commonly marked with wavy marginal lines. 



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

 feebly elevated glabella. 



Fam. 2. llUvnicke. — Pleime unfurrowed. Pygidium with smooth sides and smooth or feebly furrow- 

 ed axis. Typ. gen. : JZtenws, Up. Cam., Sil.; Bumastus,H\\.] i\'i7eMS, Lr. Sil. ; Stygina,'Lv. ^\\. 



Fam. 3. AsapMdœ. — Pleuiœ furrowed. Pygidium with smooth sides and smooth or feebly furrowed, 

 axis. Typ. Gen.: lUœnvrus, Cam.; Niobe, Lr. Sil. ; Asaplant, Lr. Sil. ; Psilocephalus, Cam.; 

 Barrandia, 1a\ Sil.; Bathyurcllus, Lr. Sil.; Bathyurus, Cam..,!^}-. Si\.; Megalaspis,ljr. Si\. 



Note. — The two fiimilies of this section, although widely se];)arated in the classification of 

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

 the fixct that certain genei-a — lllœnurus, Niobe and Stj'gina, more especially — apfiear to form an 

 intermediate or transition grouj). But the t^'pical representatives of each familj^ are clearly charac- 

 terised, and connecting points of this Icind are abundantly traceable in the case of other families 

 universally regarded as distinct. In the present classification, indeed, the collocation 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 Asaphidœ proper the forms with horned head- 

 shield and strongly furrowed pygidium, 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 sti-ongly furrowed, lobate and conspicuously horned 

 forms, to the smooth IllsenidiC and Asaphidic. The horned genus, Megalaspis, forms the connecting 

 link. 



§ 2. SuLCATi. — Broad-formed or oval ti-ilobites with horned head-angles, pointed pleiinc, and large, 

 strongly-furrowed pygidium. 



Fam. 4. Basilicidœ — C41abella with short side-furrows, or smooth. Typ. gen. : Basilicus {=Asaphi)s 

 in part), Lr. Sil.; Oxijgia, Lr. Sil.; Homalotopeon, Lr. Sil. 



Fam. 5. Dikelocephalidœ. — Glabella strongly developed, with transverse or 8i<ie furrows. Number 

 of body-segments unknown. Tyjî. gen. : Difcelocephalus, Gam.; Dorypyge, Cam. 



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

 Cambrian forms under the liimily of the Olenida;. But fiom these it is distinctly separated by its 

 large pygidium. This latter chai'acter, together with its horned head-shield and the meeting of its 

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

 and furrowed Asaphidaj or Basilicidœ, in many of which, as well as in many species of the Asaphidio 

 proper, the course of the suturai outline is the same. The greater number of bod3^-segments may 



