§ 21. 



SILURTAN TRILOBITES. 791 



met with ; the detached segments of which might easily be 

 mistaken for remains of Chelonian reptiles.* The body is 

 of an oval shape, and the posterior angles of the head are 

 rounded : the thorax is composed of eight segments. 



A remarkable kind of Trilobite occurs in the Ludlow 

 limestone, and has been named Homalonotus {Lign. 181) 

 by Mr. Konig, from the almost entire absence of the lateral 

 furrows. The surface is scabrous, and the thoracic portion of 



Lign. 181. — Trilobite from the Ludlow rocks. 



{Homalonotus delphinocephalus ; reduced from PL VII. Sil. Si/st.) 



the carapace, which is but obscurely lobed, consists of thirteen 

 segments ; the abdominal region is distinct from the thoracic, 

 and formed of nine rings : it terminates in a prolonged point. 

 In the late work of an eminent naturalist, M. Burmeister,f 

 the trilobites are arranged under fifty genera, comprising 

 250 species ; mrmy of the species named by former ob- 

 servers being grouped together, from their proving to be 

 the same animals in various states of metamorphosis. The 



* Dr. John Locke, of the Medical College, Ohio, who first developed 

 the structure of this gigantic Trilobite, has obliged me with specimens 

 that leave no doubt as to the fidelity of his model of the perfect form 

 of the original, of which there is a specimen in the British Museum, 

 nearly two feet in length. 



•f Die Organisation der Trilobiten : Berlin, 1843. 



