26 GEOLOGICAL MEMOIRS. 



placement which the movements m the slaty rocks have given rise to, 

 and which movements he considers to have heen sharj) and violent 

 shocks, which have broken the testaceous species ; while such as had 

 only a corneous covering (most Trilobites) were bent and distorted, 

 elongated, shortened, or laterally squeezed, according to the position of 

 the fossil, and direction of the shock. 



He describes and figures these displacements in lUcenus (/iganteus, 

 Burm., from Angers ; which he calls I. Desmaresti, and enters into 

 an argument to show, that as Guettard's figures consist only of tliis 

 species and Cali/metie, and as Brongniart refers them to his genera 

 Oyygia and Calymene, he must have intended by his O. Desmaresti 

 some of the specimens of the lllcenus. Brongniart's description, too, 

 contains references to the entire form ; he says, " une fois et demie 

 plus longue que large ; " which he could not have drawn from his 

 own figured specimen, but which would agree well with Guettard's 

 figures of Illcenus under his eye. The author would therefore take 

 away from Brongniart's species the diagnosis, and the figure by which 

 it is illustrated (and which he admits to be a good figure of the Ogygia), 

 and transfer the name, and so much of the description as will agree 

 with it, to Guettard's Illcenus^ under the above name ; giving to the 

 Ogygia the new name O. Brongniarti. 



Some additional notes are given on Trinucleus, first with reference 

 to the propriety of Burmeister's division of Trilobites into those which 

 have, and those which are deprived of the power of rolling up, as he 

 believes that in this genus both divisions may be found ; T. granu- 

 latus having the thorax-joints all soldered, while in T. Pongerardi 

 the rolling is evident, and even twice upon itself — a condition 

 more complete than in any other Trilobite. He mentions also that 

 Green in 1832 showed that T. tesselatus could bend. He also goes 

 over the history of the genus, and points out that Wahlenberg, Green 

 and Corda describe the fringe without understanding its true structure, 

 the nearest approach to a true description of it being found in the 

 " Silurian System," where the name " Tretasjns,^^ or perforated shell, 

 is spoken of as appropriate for it. Beyrich, in 1846, had called the 

 points 'alveoli,' and Barrande 'points.' He then refers to his own 

 previously published account of its true nature as a hollow double 

 crust, perforated by cylinders of shell, previously given in the " Bulle- 

 tin," and proposes T. Pongerardi as the type of the genus. 



A new species of Calymene, C.AragOy allied to C. BlumenhacTiiiy but 

 the tail with a many-jointed axis and smooth sides, is described and 

 figured.— [J.W.S.] 



On the Natron Lakes in the Plain of the Araxes. 

 By Prof. H. Abich. 



[From Bull. del'Acad. de St. Petersbourg, vol. v. p. 117.] 



One of the least important of these lakes occurs about two 

 wersts from the Armenian village Tasch-burun {Stone-nose) at the 

 extremity of the extensive lava-mass, which in one of the most recent 



