TRIL0B1TA. 



533 



Calymene itself (fig. 372) 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 quadrate and forked posteriorly. The 

 animal possessed in perfection the 

 power of rolling up into a ball, speci- 

 mens commonly occurring in this 

 condition. The species of Calymene 

 are principally Ordovician and Silu- 

 rian, and the three most familiar 

 species are the nearly allied C. Blu- 

 menbachii, C senaria, and C. brevi- 

 capitata. The genus is also known 

 to have survived, in North iVmerica, 

 into the commencement of the De- 

 vonian period. 



The genus Homalonotus is the 

 only other member of the Caly- 

 me?iid<z, and is distinguished from 

 Calymene by the greatly elongated 

 and faintly trilobed body (fig. 389). 

 The glabella, further, is smooth or 

 exhibits but faint traces of lobation. 

 The species of Homalonotus are Si- 

 lurian and Devonian, and the genus 

 has a wide distribution in space. A 

 very familiar species is the Ho?nalo- 

 notus delphinocephalus of the Silurian 

 rocks. 



Family 7. Asaphid^e. — Large 

 Trilobites, generally oval, and never 

 furnished with spines or tubercles on 

 their surface. The eyes smooth, and 



the facial sutures terminating on the posterior margin. The 

 cephalic and caudal shields generally of large size, the glabella 

 of the former often obscure, and the latter sometimes exhibiting 

 no indication of its component segments. The body-rings usually 

 eight in number, with grooved pleurae. The family is character- 

 istically Ordovician, and the two principal genera are Asaphus 

 and Ogygia. In the genus Asaphus (figs. 390, 392, 393) the 

 general trilobation is somewhat indistinct, and the caudal shield is 

 generally equal to the head in size. The genal angles of the head- 



389.— Homalonotus delphino- 

 cephalus. Silurian. 



