170 



ANNULOSA. 



development in the Silurian period. The chief genera are 

 Cheirurus, Amphion, Sph&rexochus, Staurocephalus, Deiphon, 

 Encrinurus, and Cybele. 



6. CALYMENID^:. Crust granulated, often tuberculated. 



Body-rings thirteen in number. 

 Facial sutures ending at the 

 posterior angles of the cephalic 

 shield. Body sometimes very 

 indistinctly trilobed (Homalono- 

 tus). The family appears to 

 commence at the base of the 

 Lower Silurian series or at the 

 summit of the Upper Cam- 

 brian, and ranges into the De- 

 vonian. The only genera are 

 Calymene and Homalonotus, 

 of which the former is mark- 

 edly trilobed, but the latter 

 very indistinctly so (fig. 116). 

 The best -known species are 

 Calymene Blumenbachii, which 

 ranges from the Caradoc (Lower 

 Silurian) to the Ludlow Rocks 

 (Upper Silurian), and Homa- 

 lonotus delphinocephalus, which 

 is a well-known Upper Silurian 

 Trilobite. 



7. PHACOPID^:. "Eyes 

 largely faceted, the cornea con- 

 vex over each facet, forming a 

 granulated, not a smooth eye. 

 Facial suture ending posteri- 

 orly on the outer margin of 

 the cheek. Thorax with eleven 

 rings " (Salter). This family includes the single genus, (phacops, 

 divided into the sub-genera Trimerocephalus, Phacops, Acaste, 

 Chasmops, Dalmania, and Cryphceus. The family Phacopida 

 ranges from the base of the Lower Silurian series (perhaps from 

 the Upper Cambrian) to the summit of the Devonian series. 

 Amongst the Upper Silurian . species may be mentioned 

 Phacops caudatns and P. Downingitz, as well-known fossils ; 

 whilst P. latifroiis is a familiar and widely-distributed Devonian 

 species. 



8. LICHAD/E. Cephalic shield small, the facial sutures 

 cutting its outer margins. Glabella large, deeply furrowed. 



Fig. 1 16. Homalonotns delphinoce- 

 plialus. Upper Silurian. 



