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SILURIAN TRILOBITES. 



expanded form, with the segments all distinct, through others which have them more or 

 less obscure, to such convex forms as Nileus and lllanus, where they are obsolete, or 

 only visible on the interior of the crust. The margin of the tail is not unfrequently 

 sharply striated, and the incurved portion (caudal fascia) is broad, distinct, and strong, 

 and often convex beneath. 



The hypostome varies in character. In most of the genera it is continuous, or in 

 some species of Asaphus it has a vertical suture only ; the rostral shield being in this 

 case atrophied. In Illcenus the rostral shield is very large and transverse. Indeed, this 

 genus is abnormal in many characters, and leads the way to the next family, No. 12, in 

 which the form is more compact, the body-segments more numerous, and the rostral 

 shield a necessary part of the structure. The labium varies much in shape in the different 

 genera, and only in this group is it occasionally forked : but it is constantly so in Asaphus. 



Having said so much on the general characters, we may proceed to describe one of 

 the most typical and constant of the genera. Ogygia is a very limited genus ; it has all 

 the elements of the Asaphida, and has a great resemblance to some of the sub-genera of 

 Asaphus, which genus has, on the contrary, the greatest variety of forms within a single 

 generic group. 



lllanides. 

 Illsenus. 

 Ilkenopsis. 



12. PEOETIDiE. 



9 — 10 rings, axis 

 elongated. 



11. BRONTEIDiE. 



10 rings : tail-axis / 

 shortened. / 



Stygina. 



Barrandia. 



io. asaphus. 



Ogygia. 



Ptychopyge. 



Niobe. 



13. TRINUCLEIDiE. 



G body-rings. 



