528 



CRUSTACEA. 



maximum of development, the leading families being the Asaphidce, 

 Phacopidce, Trinudeidce, Cheiruridce, and Calymenida, and the chief 

 genera being Asaphus, Ogygia, Phacops, Trinucleus, Ampyx, Cheiru- 

 rus, Encrinurus, Calymene, and Homalonotus. In the Devonian 

 rocks, again, Trilobites are tolerably abundant, though less so than 

 in the preceding series. The commonest Devonian genera are 

 Phacdps, Homalonotus, Proetus, and Brojtteus. Lastly, the order 

 seems to die out before the close of the Palaeozoic epoch, being 

 represented in the Carboniferous period solely by the four genera 

 Phillipsia, Brachymetopus, Griffithides, and Proetus ; while in the 

 Permian rocks only a single species of Phillipsia has hitherto been 

 detected. 



In the following, a brief summary of the families of the Trilobita, 

 indicating the principal genera of each, and their distribution in 

 time, will be given. No strictly zoological arrange- 

 ment of these families is as yet possible, except in a 

 general sense, but the classification proposed by M. 

 Barrande, one of the most illustrious palaeontologists 

 of this century, has been in the main adhered to. 



Family i. HarpedidtE. — Cephalic shield large, 

 and horse-shoe-shaped, its posterior angles greatly 

 prolonged, and its margin or "limb" perforated by 

 pores (fig. 383). The glabella is conical and pro- 

 minent, with slightly marked furrows, and the eyes 

 are small, and consist of a few lenses. There are from twenty-five 

 to twenty-nine thoracic segments, and the pygidium is extremely 

 small, and consists of three or four amalgamated segments. The 

 species of Pfarpes, the sole genus comprised in 

 this family, are principally Ordovician and Silu- 

 rian, but a few Devonian forms are known. 



Family 2. Remopleurid^e. — In this family the 

 head is greatly developed, semicircular in shape, 

 the genal angles produced into spines. The 

 glabella is smooth, or possesses three pairs of 

 lateral grooves, and the facial sutures unite in 

 front of it. The eyes are very long and are 

 reticulated. There are eleven to thirteen body- 

 rings, with grooved pleurae, and the pygidium is 

 very small, and is often reduced to two segments. 

 This family contains only the single genus Remo- 

 pleurides (fig. 384), which is confined to the Ordo- 

 vician and Silurian deposits. 



Family 3. Paradoxid^e or Olenid^e. — Head-shield well devel- 

 oped, crescentic, the genal angles produced. The glabella, typi- 

 cally, widest anteriorly, with well-marked grooves. The facial 



Fig. 383.— Harpes 

 ungiila, from the 

 Silurian of Bohemia. 

 (After Barrande.) 



Fig. 384. — Rcmo- 

 filcurides radians. 

 Ordovician. (After 

 Barrande.) 



