792 THE WONDERS OF GEOLOGY. Lect. VIII. 



Trilobites are supposed by M. Burmeister to have inhabited 

 deep water, near the sea-coasts, to have swam with facility, 

 and with the back undermost. As no traces of legs have 

 been discovered, it is probable that they possessed soft 

 perishable paddles, bearing branchiae. Their food consisted 

 of marine animalcules; they were gregarious, grouping toge- 

 ther in innumerable multitudes. Like other articulata, they 

 cast their shells or cases ; hence the abundance of their exu- 

 viae . Mr. Burmeister is of opinion that they were most nearly 

 allied to the existing family of crustaceans, the Phyllopoda. 



The geological distribution of the different genera of 

 Trilobites in the subdivisions of the Silurian system, though 

 presenting some anomalies, is considered by Sir R. Mur- 

 chison as well defined. According to this eminent geologist, 

 the Homalonotus is characteristic of the highest zone, or 

 Ludlow rocks, being most abundant in the Wenlock and 

 Ludlow limestones : the Calymene ranges through those 

 deposits, but is particularly abundant only in the lower 

 limestones, beneath which it has not been observed. The 

 Asaphus caudatus extends from the Ludlow rock to the base 

 of the Wenlock formation : both these forms are, therefore, 

 characteristic of the Upper Silurian. In the Lower Silurian 

 the genera Trinucleus,* Agnostus,f and Ogygia,J are the 

 prevailing organisms of this class. § 



22. Visual Organs of the Trilobites. — The eyes of 

 the Trilobites resembled in structure those of crustaceans 

 and insects, which are composed of a vast number of elon- 

 gated cones, each having a crystalline lens, pupil, and cornea, 

 and terminating on the extremity of the optic nerve. Each 

 organ of sight is, therefore, a compound instrument, made 



* Medals of Creation, p. 558. A memoir, On the Structure of the 

 Trinucleus, by Mr. Salter, is published in Geol. Journal, vol. iii. p. 251. 



f Ibid. p. 565. J Ibid. p. 559. 



§ See, On the Geological Distribution of Fossil Crustaceans 

 Medals of Creation, p. 503. 



