ROUAULT ON TRILOBITES. 25 



thorax being nearly uniform throughout a genus, assist him very 

 much in the restoration from fragments. He finds that Ogygia has 

 the body nearly equally divided into three— the middle of the length 

 being between the 4th and 5th segment ; while in Calytnene, Homa- 

 lonotus and Paradoxides, the thorax occupies more than half the 

 entire length, the centre being at the 6th segment ; in Illcenus it is 

 betw^een the 5th and 6th. In Asaphus and Nileus it is between the 

 4th and 5th ; in Trinucleus at or above the 1st. 



Again, in Ogygia the greatest width of the oval body is in the 

 centre, or between the 4th and 5th segment. 



By all these observations of the species he is guided to the same 

 result, and therefore believes that the restoration he has given is not 

 only correct in the present instance, but may form the basis of future 

 restorations where fragments only exist ; and even where slaty distor- 

 tions may have altered the original form entirely, the relative pro- 

 portions will still remain intact. 



Abbreviating a little, O. Brongniarti is thus described : — 

 " Slightly oval ; the width rather greater. Head crescent-shaped, 

 Z\ times as wide as long. Glabella wide, short, only |ths the 

 length of the buckler, with two shallow furrows radiating inwards 

 from the upper angle of the eye, one obliquely upwards, the other 

 horizontal. Cheeks flat, triangular, not separated from the gla- 

 bella. Eye large, horny, with 1000 or 1200 very fine lenses at 

 least. Palpebra* continuous with the glabella. Facial suture pa- 

 rallel (within) to the front margin for some distance, then sharply 

 turning round backwards to the eye, and thence outwards to the 

 very end of the cheek before it curves inwards again to cut the 

 posterior margin. 

 " Thorax wide ; 8 segments, oblique and curved back at the ends ; 

 pleurae once and a half the width of the axis, with diagonal fur- 

 rows. 

 " Tail (as restored) wider than semicircular, wdth a broad conical axis, 

 and a margin only moderately wide." 

 The characters of O. Edwardsi, Rouault, are given : — 

 *' Oval, less elongate than O. Biichii. Head semicircular, 2\ times 

 as wide as long ; margin and posterior spines much-dilated, and 

 covered with fine striae ; glabella longer than broad, without fur- 

 rows, convex in front, the convexity continued, but narrower, to the 

 faint neck-furrow ; cheeks small, flat. 

 " Eyes and facial suture nearly as in O. Brongniarti, the suture less 

 parallel to the edge in front. Thorax one-third wider than long ; 

 8 joints, each with a gently curved diagonal furrow, and obliquely- 

 pointed and curved back at their ends. Tail with wide lateral 

 lobes, the axis of 18 joints (with a backward notch in each), late- 

 ral lobes 8-ribbed ; border concentrically striate." 

 He endeavours to show that Ogygia had the thorax-joints soldered, 

 and not therefore capable of bending or of slipping under each other, 

 although they were not so firmly united as to resist the lateral dis- 



* A useful term for the upper lunate plate which covers the eye in most Tri- 

 lobites. 



