SUBCLASS I 



TRILOBITA 



709 



and the latter adopts a division into two lines, based on the number of body- 

 rings and size of the pygidium. These include and are themselves included 

 in four groups, founded on the presence and form of the facial suture and the 

 structure of the eyes. (13) Chapman, in 1889, proposed four suborders or 

 primary groups based purely upon arbitrary features of general structure and 

 configuration, especially the form of the glabella, whether wide, conical or 

 enlarged. (14) Haeckel, in 1896, divided the Trilobites into two orders 

 based upon the presence or absence of a functional pygidium. 



The classification here followed is essentially that prepared by Beecher for 

 the first edition of this treatise, but with some amplification and modification, 

 made possible by the recent work of Reed, Clarke, Jaekel, Walcott and others. 

 Beecher's classification, although not universally accepted, has proved superior 

 to any previously proposed, and forms the basis for most of the modern 

 investigation of the group. 



Order 1. HYPOPARIA Beecher. 



Free cheeks forming a continuous marginal ventral plate of the cejihalon, and in 

 some forms also extending over the dorsal side at the genal angles. Siiture ventral, 

 marginal or snlmarginal. Compound paired eyes absent ; simple eyes may occur on each 

 fixed cheek, singly or in pairs. 



Even in the higher genera of this order, the suture is frequently imnoticed, but can 

 be seen in all well-preserved specimens. In Gryptolithus and Harpes it follows the edge 

 of the cephalon, and separates the dorsal from the ventral plate of the pitted brim. 

 Since eye-spots occur on the fixed cheeks in the young of Gryptolithus and adult 

 Harpes, it is probable that this character is a primitive one in the order, and has 

 been lost in Agnostus, Eodiscus, 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 of 



Fio. 1357. 

 Cepliala of //(/yiojiana. A, Agnostus. B, Eodiscus. C, Harpes. D, Crypiolithus. E, Ampyx (after Beecher). 



the cephalon. Therefore the Agnostidae, Cryptolithidae, and Harpedidae have, in 

 this respect, 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 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 Proiitus, 

 Asaphus and Lichas. The pygidium often fails to indicate its true number of 

 segments. Many species of Agnostus and Eodiscus show no segments either on the 

 axial or pleural lobes of the pygidium. Gryptolithus and others may have a numerously 

 annulated axis and fewer grooves on the pleural portions. The number of ajipendages 

 corresponds to the axial divisions. The multiplication of segments in the laygidium, 

 and their consequent crowding, make them quite rudimentary. 



