624 ARTHROPODA SUB-KINGDOM VIL 
marginal, or submarginal. Compound paired eyes absent ; simple eyes may occur on each 
free cheek, singly or in pairs. 
Even in the higher genera of this order, the suture is frequently unnoticed, but can 
be seen in all well-preserved specimens. In TJrinucleus and Harpes it follows the edge 
of the cephalon, and separates the dorsal from the ventral plate of the pitted limb. 
Since eye-spots occur on the fixed cheeks in the young of Trinucleus and adult Harpes, 
it is probable that this character is a primitive one in the order, and has been lost in 
Agnostus, Microdiscus, Ampyxz, and Dionide. 
The ontogeny of higher genera shows that the true eyes and free cheeks are first 
developed ventrally, appearing later at the marginal, and then on the dorsal side of 
the cephalon. Therefore the Agnostidae, Trinucleidae, and Harpedidae have a very 
primitive head structure, characteristic of the early larval forms of higher families. 
Other secondary features show that this order, though the most primitive in many 
A L Cc 
Fic. 1281. 
Cephala of Hypoparia. A, Agnostus. B, Microdiscus. C, Harpes. D, Trinucleus. E, Ampyx (after Beecher). 

respects, is more specialised than either of the others, except in their highest genera. 
The characters referred to are the glabella and pygidium. Very few species show the 
primitive segmentation of the glabella, it being usually smooth and inflated and 
resembling in its specialisation such higher genera as Proétus, Asaphus, and Lichas. 
The pygidium often fails to indicate its true number of segments. Many species of 
Agnostus and Microdiscus show no segments either on the axis or limb of the pygidium. 
Trinucleus and others may have a numerously annulated axis and fewer grooves on the 
pleural portions. The number of appendages corresponds to the axial divisions. The 
multiplication of segments in the pygidium, and their consequent crowding, make them 
quite rudimentary. 
Family 1. Agnostidae. Dalman. 
Small forms, having the cephalon and pygidium elongate, nearly equal, and similar 
in form and markings. Free cheeks ventral, continuous ; suture marginal 
or ventral. Eyes wanting. Thorax composed of from two to four seg- 
ments, with grooved pleura. Cambrian and Ordovician. 
Some species have spines at the genal angles, corresponding to the 
interocular spines of Holmia and young 
Elliptocephala, and not to the spiniform 
projections of the free cheeks, From their 
abbreviated thorax, and progressive loss of 
annulations on the glabella and pygidial 
axis, they must also be considered as de- 

Fic. 1282. Fig. 1283. graded.  Microdiscus, the earliest genus, 
ssnosins granu- Agnostus pisiformis, Linn. shows in some species the normal penta- 
atus, Barr. Cain- Cambrian ; Andrarum, Sweden. 4 4 . * r : ea 
briun; Skrey, Complete individual, and frag. Merous glabella, and annulated pygidial 
Bohemia. 5/; ment of limestone with detached axis, while the later genus Agnostus has lost 
(after Barrande). cephala and pygidia. , é : malls 
the annulations of both axis and pygidium. 
Agnostus, Brongn. (Figs. 1281, A-1283). Cephalon and pygidium without annula- 
tions on their axes; thoracic segments, two. Extraordinarily abundant in the Middle’ 

