Crusracea. | LOWER PALZZOZOIC ARTICULATA. 141 
Ist Subfamily. AGNOSTIN 2. 
Minute, blind, head and tail covered by nearly equal and similar rotundato-quadrate shields ; only two 
thoracic segments. 
This subfamily includes both the families Phalacromides and Battoides of Hawle and Corda which only 
differ in the margin of the tail of the latter being denticulated, and that of the former being smooth,—points in 
my mind scarcely of generic importance. 
From the absence of eyes and the very slight powers of locomotion argued by so small a number of 
feet bearing rings, I strongly suspect that the genera of the present family were parasitic, and may hold the 
same position among the Trilobites that the Swcforia do among the crustacea generally, that group being 
distinguished from its allies by the want of eyes, few body rings, little or no powers of locomotion, abnormally 
and variously shaped bodies, and being parasitic (generally) on fish. Bophyrus, the analogous group among 
the Lsopoda is usually parasitic on the gills of Crustacea under their carapace, and such I strongly 
suspect were the habits and mode of life of the Agnostinw, living in all probability attached to the gill 
feet on the under side of Trilobites, the largest known species of which accompany the little Agnosti. 
Genus. TRINODUS (J/°Coy). 
Syn.— = Arthrorachis Hawle and Corda. 
Gen. Char.—Cephalic and pygidial shields nearly equal, semielliptical (or from the projection of the 
middle of the articular margin, obscurely pentagonal) ; g/abel/a semicylindrical, strongly defined, having a very 
minute lobe at each side of the base; cheeks convex, horse-shoe shaped, entirely surrounding the glabella ; 
external margin narrow, defined, convex, not extended at the angles; pygidium having an elongate tubercle 
down the middle of the axal lobe, which is strongly defined, semicylindrical, and divided on each side of 
the mesial tubercle by two fine transyerse sulci; lateral lobes horse-shoe shaped, convex, entirely surrounding 
the axal lobe, surrounded by a distinct external margin: surface smooth. 
I originally defined this genus in 1846 in my “ Synopsis of the Silurian Fossils of Ireland,” and pointed 
out its distinctions from Agnostus (absence of the characteristic transverse divisional sulcus of the glabella, &c.) 
Subsequently (1847) Hawle and Corda figured and described the group under the title «rthrorachis, and 
likewise distinguished it from Agnostus, or Battus, without knowledge of what I had done. By some curious 
mistake however, Mr Salter has stated in the ‘“‘ Mem. of the Geol. Survey,” (Vol. II. Pt. 1. p. 351) that 
I have only described the tail in my generic character, and also overlooking the independent establishment of 
the same genus by the Bohemian Palzontologists, has reunited it to Agnostus. 
TRINODUS AGNOSTIFORMIS (J/°Coy). Pl. 1. E. fig. 10. 
Trinodus agnostiformis (M°Coy), Syn. Sil. Foss. Irel. t. 4. f.3. Agnostus trinodus B convexus 
(Salt.) Mem. Geol. Surv. Vol. II. p. 350. 
Sp. Ch.—Cephalie shield semioval, about one-fifth longer than wide; glabella sharply defined, very 
conyex, narrow, semicylindrical, front rounded; cheek-piece horse-shoe shaped, convex, width equal at the 
sides and round the front, slightly exceeding that of the glabella (? the front divided by a faint longitudinal 
mesial furrow) ; outer margin about half the width of the cheeks, convex. Length of head two lines, length of 
glabella one and half lines, width of glabella rather more than half a line. 
I have some doubt whether the longitudinal furrow in the middle of the front of the cheek-piece be 
correctly observed, as I have only one specimen of the species at hand for examination, and if constant 
it must have escaped me at first. The tail has been figured in the above volume of the “ Memoirs of 
the Geol. Survey,” but the segmental furrows seem to have been overlooked. 
Position and Locality—One specimen, Limestone of Rhiwlas, Bala, Merionethshire. 
Explanation of Figure.—PI. 1. E. fig. 10. Head natural size, and magnified six diameters; the dotted 
line indicates the portion of the margin which is wanting. From Rhiwlas. 
