146 Natural-Historical Collections. 
pure minerals have not always a specific gravity as uniform as we should be dis- 
posed to expect, from the importance of this character. Carbonate of lime, for 
example, varies between 2.7 and 2.5, arragonite between 2.9 and 2.7, &c. Their 
state of crystallization influences them in a sensible manner. The specifie gra- 
vity is always greatest in little crystals; in large crystals it diminishes, probably, 
because they have in their interior cavities more or less considerable, even when 
the mass appears to be most homogeneous. The varieties with lamellar structure, 
or the fibrous, are lighter, and that in proportion to the size of their lamme ; 
finally, it is in the varieties which result from decomposition that the specific 
gravity is not diminished. But it is very remarkable that in each substance, the 
difference between the two extremes is sensibly of the same value; and all the 
varieties of the same substance return to the same specific gravity, when they are 
reduced to powder, proving that the variations are only connected with cavities 
in their substance. It is thus alone that we can make specific gravity a compa- 
tative character, and consequently of certain importance in mineralogy. 
The Animal of the Siliquaria.—_M. Audouin has announced to the Philoma- 
thic Society of Paris, the discovery of the animal of the Siliquaria, whose tube 
alone was before known. This genus, confounded by Linnzus with the serpula, 
was established by Bruguiéres, upon characters derived from the shell. The 
Siliquaria has hitherto been generally referred, by supposition, to the sedentary 
division of the class Annelida, but M. Audouin places it amongst the Modlusca, 
in which class it approximates to the Vermetus of Adanson. It is furnished with 
a very thick, horny, operculum. Its mantle is cleft from one extremity to the 
other, and, contrary to the assertion of M. de Blainville, the branchie exist only 
-on the left side. The body curls upon itself at its anal termination. The head, 
which is distinct, is provided with two eyes situated at the base of a pair of cylin- 
drical tentacula, slightly enlarged at their summit. 
Xanthus Desmaresti ; anew species of Fossil Crustacea.—To the thirty- 
four species of fossil crustacea described by M. Desmarest, M. Polydore Roux 
adds, from his collection, one which he attributes to the same formations in the 
East Indies as are characterized by the Grapsus apna many Gonoplaces, and 
other species so numerous in the cabinets. 
The genus Xanthus, established by Leach, does not differ essentially from the 
“crabs, except in the position of the external antenne, which, instead of being 
placed between the internal canthus of the eyes and the forehead, are inserted in 
the depth of the internal canthus. It belongs to the order Decapoda, family Bra- 
chyura, and tribe Arcuata of Latreille. 
The characters of this new species (the male of which is described) are : 
““ Shell gibbous, having its regions highly embossed, and its lateral margins 
granulated and festooned; orbits little separated. The sternal parts granular ;, 
feet smooth ; length, 0.030 ; breadth, 0.042.” 
The colour is brown, and M. Roux’ specimen is crusted with an argillaceous 
paste. The general form presents some analogy with the Cancer floridus of 
Herbot, pl. 21. fig. 120, which is itself a Xanthus.Ann. des Sciences Nat. 
May 1829. 
Nidification of the Tortuga.—The Tortuga or large fresh water turtle travels 
far at times. It deposits its eggs in the sand witli surprising address. The land 
turtles, it is said, are most stupid.in this particular, dropping their eggs, one by 
-one, as they hobble over the ground, neither covering nor taking any care of them 
whatever, nor paying any regard to their offspring. The tortuga, on the contrary, 
covers its eggs so accurately as to leave no signs perceptible of its nest; and, 
- however strange it may. seem, she so arranges it as to make her track appear 
unbroken over the sands, and, after laying her eggs, she proceeds on again in the 
same direction to complete ‘the deception._—_Edin. Journ. of Scicnagt New 
Ser. I. 244, 
