TEILOBITA. 



483 



fossil state, especially in the Sohlenhofen lithographic slate, but also 

 in older formations as far back as the Uebergangsgebirge (Cambrian, 

 Silurian, etc.) formation. 



Limulus moluccanus Latr., East Indies. L. polypJicmus L., East Coast of 

 North America. 



TRILOBITA.* 



In connection with the Merostomata and the Xiphosura, the 

 Trilobites may be considered. Their systematic position cannot as 

 yet be denned with certainty. 

 They lived only in the most an- 

 cient periods of the earth's his- 

 tory, and their fossil remains are 

 found in great numbers and are 

 excellently preserved; but, un- 

 fortunately, the conditions under 

 which they were fossilised were 

 such that the under side of the 

 body, and, consequently, the 

 structure of the appendages, that 

 is the very characters which 

 would enable us to decide their 

 affinities, remain unknown to 

 us. We may probably infer 

 from this absence of any trace of 

 appendages * in the fossils, that 

 the legs were soft and delicate ; 

 but Burmeister's conclusion that 

 they resembled the legs of the 

 Phyllopoda is not justified. 



The body, which is frequently 

 found rolled up, is covered with 

 a thick shell, which is divided by two parallel longitudinal furrows 

 into an elevated median portion (rhachis) and two lateral portions- 

 pleura): it rarely attains any considerable size. There is an 



* Burmeister, " Die Organisation der Trilobiten," etc., Berlin, 1843. Beyrich, 

 "Untersucmmgen liber Trilobiten," Berlin, 1845, 1846. J. Barrande, "Systeme 

 silurien du centre de la Boheme," Prague, 1852. S. W. Salter, "A monograph 

 of the British Trilobites," London, 1864-1866. 



* Portions of appendages have been recently observed on the ventral surface 

 of an Asaplins ( k< Notes on some Specimens of Lower Silurian Trilobites, " by 

 E. Billings : also "Note on the Palpus and other appendages of Asaphus," etc., 

 by H. Woodward, Quart. Jmirn. of the Gcolog. Soc., London, 1870), which are 

 said to point to the affinity of Trilobites with the Isopoda. 



FIG. 376. Din gram of Dalmat\tv* (after 

 Pictet). 61, Glabellum ; Sf, great suture 

 (ocular suture); 0,eyes; GP, separable gena 

 (cheeks) ; Kh, rhachis (tergum) ; PI, pleu- 

 ron ; Pg, pygidium. 



