624 



ARTHROPODA 



SUB-KINGDOM VII 



tintrginal, 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, hut can 

 be seen in all well-preserved specimens. In. Trinucleus 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 prohahle that this character is a primitive one in the order, and has been lost in 

 Agnostus, Microdiscus, Ampyx, 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 <>!' 

 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 





FIG. 1281. 

 Cephala of Hypoparin. 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 Proetus, 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. Dalinan. 



Small forms, having the cephalon and pygidium elongate, nearly equal, and simi/nr 

 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 spinifonn 

 projections of the free cheeks. From their 

 abbreviated thorax:, and progressive loss <!' 

 annulatioiis on the glabella and pygidial 

 axis, they must also be considered as de- 

 graded. Microdiscus, the earliest genus, 

 shows in some species the normal penta- 

 merous glabella, and annulated pygidial 

 axis, while the later genus Agnostus has lost 

 the annulatioiis of both axis and pygidium. 

 Agnostus, Brongn. (Figs. 1281, yl-1283). Cephalon and pygidium without annula- 

 tions on their axes ; thoracic segments, two. Extraordinarily abundant in the Middle 



Flu. 1282. 



Agnostus grami- 

 lattts, Barr. Cam- 

 brian; Skrey, 

 Bohemia. 6/j 

 (after Barrande). 



Agnostu ,./>;/,,///'.,, Linn. 

 Cambrian ; Aiidrarum, Sweden. 

 Complete individual, and frag- 

 ment of limestone with detached 

 cephala and pyjjidia. 



