ON TRILORITHS. 117 



prove to bo a divergent character, but tlie actual number of llie.se is at present uncertain, and in the 

 closely related Dorypyge the thorax is entirely unknown. 



4^ .".. Palmati. — Broad-formed triiobites, wilh lari^e, .short-axcd, radiatei3'-furro\ve(l pygidium. 



Fani. ('). Lichasichv. — Pleuia: fmrowcd. T'ygiciium with short, broail axi.s and deeply indented 

 margin. Typ. gen. : Lichas, Sil. 



Fam. 7. Bronteido'. — Pieurai wiih slightly raised surface-band. Pygidium with veiy short axis and 

 entire margin. Typ. gen. : BTOH/ci/s, Lr. Sil, Dev. 



§ 4. CoLUM.VATi. — ^Trilohites of more or less oval form, with largo ]iygidium, the axis of which is 

 l3-pically long, promiricnt and many-segmented. 



Fam. 8. PAaefon/(/a'.—ti label la short, conical. Body-axis comparatively narrow. Pygidium in 

 some cases with digitulcd outline. T^'p. gen.: Phaeton, Sil., Dev. ; Dcchnnella, Dev. ; 

 Brachtjmetopus, Carb. 



Fam. 0. Proctidtv. — Glabella lai'ge. smooth or furrowed. Body-axis comparatively broad. Axis of 

 iiygidiura typically wilh 10 or moie pseudo-segments. Typ. gen.: Proetus. Sil., Dev.; 

 Phillipsia, Dev., Carb. ; Griffiihides, Carb. 



Note. — The famil}' of the Proetida', as commonly recognized, includes under the genera Proetus 

 and Phillipsia forms of marked dissimilarity jf a.'<pect, so far, at least, as regards the glabella and to 

 some extent the i-elative width of the body-axis. Whilst in Pioetus proper the glabella is very large, 

 reaching quite, or very nearly, to the anterior margin of the headshield, in the forms referred to the 

 sub-genus Phaeton (= Proe'usstrkitus, Barr.) it is remarkably short and small. The digitated margin 

 of the pygidium, sometimes referred to as a sub-generic distinction, is a character of quite subordinate 

 value. In Deehenelhi and in Brachymotopus, commonly placed under Phillipsia, there is a similar 

 departure, as regards the glabella, from the typical generic form '. I have, therefore, separated these 

 tj'pes with short glabella from the genera with which they are commonly associated, and have placed 

 them in a distinct family. 



Sub-Order III. — CONIFRONTES.— Triiobites of more or less elongated form, tapering downwards. 

 Body-axis of 13 or more segments, not distinctly separated fi'om the pseudo-segments of the 

 axis of the pygidium. The latter, typically, of moderate size, or small. Glabella : -coniform, 

 contracted (or not expanded) anteriorly. 



§ 1. LoNGicoNi. — Glabella comparatively lai-ge, extending to, or nearly to, the anterior margin of 

 the head-shield. Facial sutures terminating at the genal angles. 



Fam. 10. Somalonotidcc. — Longitudinal furrows feebly indicated. Glabella: flat, typically unfiirrow- 

 ed. Typ. Gen.: Homalonotus, Sil., Dev.; Burmeisteria {= H. arnirtfus and other spinose 

 forms), Dev. 



Fam. 11. Cali/menidd'. — Thorax, pygidium and glabella, strongly lobed. The glabella tapering 

 anteriorly. Typ. gen.: Cal y mene, Si\.; JVeseuret us, Cam. 



Fam. 12. Triarthrida'. — Glabella of nearly uniform width, with side furrows. Body segments 14 to 

 15 in number. Typ. gen. : Triarthrus, Lr. Sil. 



Note. — Homalonotus, commonly placed under the Calymenida>, is hero separated on account of 



' Ko doubt a series of intermediate varieties might bo found in which tlio distinction becomes gradually 

 lessened or obscured ; but connecting series of tliis kind may be traced more or les.s everywhere, and if rigidly 

 followed out woukl render generic distinctions practically impossible. 



