24 GEOLOGICAL MEMOIRS. 



tany," an abstract of which appeared m vol. iv. part 2, p. 35 of the 

 Quarterly Journal of the Geological Society. 



The comparative amount of calcareous matter in the test of Trilo- 

 bites, and the shells of Mollusks, may certainly be determined, where 

 the sulphuret of iron has been abundant, as at Angers and Poligne, 

 by the amount of this mineral replacing it, since at certain spots, such 

 as Couyere, where there has been a paucity of the sulphuret, the lime 

 is found in corresponding quantity, or if the test or shell was corneous, 

 it has been replaced by sulphate of barytes. 



Among the very few genera which appear to have had a consider- 

 able quantity of calcareous matter in their test : — 



Calymene and Phacops offer either the original calcareous test, in 

 which case these fossils are very difficult to disengage from the matrix ; 

 or the sulphuret of iron. 



Ogygia and Illcenus break out with ease from a block of stone, and 

 show a varnished or lustrous surface : these do not present any sul- 

 phuret of iron (except when the mineral has been overabundant), but 

 Oyygia and Nileus"^ often have sulphate of barytes replacing them. 



A new Orthis, of the ' arcuato-striatce^ group, figured under the 

 name O. Berthoisii, Rouault, appears to have had shelly matter only 

 at the beaks, since these have not undergone distortion, are difficult 

 to disengage from the stone, and show either carbonate of lime or 

 iron pyrites ; the rest of the shell has been pressed into every pos- 

 sible form, is easily disengaged from the rock, and presents no py- 

 rites. The surface of the slate immediately round the shell is gene- 

 rally smooth and lustrous for some little distance, which the author 

 thinks may be due to the animal matter pressed out of the shell : 

 this surface is generally covered by sulphate of barytes. 



Some of the Trilobites of the clay-slate, preserved in the museum 

 of the " Jardin des Plantes," are next examined critically. 



The fine species of which Brongniart figured a portion as Ogygia 

 Desmaresti is here reproduced under the name of O. Brongniarti, 

 Rouault, and figured of full size. It is distinguished from all described 

 species by marked characters of breadth and proportions of the parts ; 

 and from O. Edwardsi, a new species also figured here, by some dif- 

 ferences of proportion more minute, and less easily recognisable. The 

 following four species are characterised by the relative proportions of 

 length and breadth in the head : — 



Length. Breadth. 



O . Brongniarti 1 3^ 



O. Edwardsi 1 2| 



O. Buchii 1 2 



O. Guettardi ..., 1 1^^ 



The author finds that in this genus — in proportion as the form of 

 the species is rounder and less elliptical — the length of the three por- 

 tions, head, thorax and abdomen, become more nearly equal ; and by 

 pursuing this clue, he is enabled to restore with great probability of 

 correctness the entire animal, and has done so in his figure. The 

 proportions borne by the head and abdomen to the length of the 

 * Of the author; it is more probably lllcenus. — [J.W.S.] 



