TRILOBITA. 



369 



proper, Acaste, Chasmops, Dalmanites, and CryplictMs) forms 

 the entire family, and ranges from the base of the Lower 

 Silurian to the Devonian, inclusive. Some of the above- 

 mentioned sub-genera may perhaps be 

 of generic value ; but they are none of 

 them separated by conspicuous charac- 

 ters from Phacops proper. 



7. CALYMENIM. In this family the 

 crust is often tuberculated or granulated; 

 the head is well developed; the glabella 

 is widest behind ; and the facial sutures 

 are discontinuous, and terminate at the 

 posterior angles of the cephalic shield. 

 There are thirteen segments in the 

 thorax, with grooved pleurae ; the tail 

 is of moderate size ; and the condition 

 of the eyes is variable. In Calymene 

 itself (fig. 210) the head is usually 

 crescentic, with rounded genal angles ; 

 and the glabella is conical, strongly 

 convex, with deep axal furrows, and 

 divided by three deep lateral grooves 



on each side, all the " lobes " thus formed being globose, and 

 the hindmost being the largest. The eyes are minute and 

 reticulated, but are rarely recognisable. The tail is convex, 

 with a well-marked axis, and the hypostome is forked. The 

 genus is found both in the Lower and Upper Silurian, and 

 the three best known species are the nearly-allied C. Blumen- 

 bachii, C. senaria, and C. brevicapitata. The genus Homa- 

 lonotus is the only other member of this family, and agrees 

 with Calymene in most respects. The glabella, however (fig. 

 226), is almost destitute of any trace of lobation, while the 

 body is greatly elongated and is but faintly trilobed. The 

 best known species is the Homalonotus delphinocephalus of 

 the Upper Silurian. 



8. PROETID^E. In this family the head is of variable 

 size, semicircular, sometimes with rounded, sometimes with 

 spinose genal angles. The facial sutures are not continuous ; 



VOL. I. 2 A 



Fig. 225. Phacops longi- 

 caudatus. Upper Silurian 

 (Wenlock) of Britain and 

 North America. 



