Natural' Historical Collections. 223 



facts in a distinct manner, than to add to the already redundant nomenclature, 



the author continues to use the names proposed. 



Before proceeding to the announcement of general propositions, he presents a 

 detailed account of the shell in several genera which differ from each other in 

 their general organization, as well as in the details of form ; and remarks that 

 having established his theory on the dilFerent sections in the collection of the 

 Jardin du Roi, of which he has analyzed almost all the genera, and in many cases 

 the greater part of the species, it will thus be easy to verify the facts alleged to 

 have discovered. 



The Chitons furnish the most positive and the most satisfactory results. They 

 are essentially distinguished from the other mollusca, by generally having eight 

 solid pieces, arranged after each other. Each of these pieces is evidently com- 

 posed of three layers, super-imposed upon each other. These three layers are 

 themselves composed of nine solid pieces. The first or outermost layer, which is 

 formed o{ a single piece, is always superior or median upon the other two, which 

 it covers more or less. In some species, as Ch. Cymba, granosus, and fascicu- 

 laris, this piece is not closely united to the second, and may even be detached 

 from it ; but in Ch. albidus, biradiatua and spinosus, it is so united. This 

 piece is the only one that exists in the Chitonellus lasvis of Blainville. It corre- 

 sponds to the basilar piece of the articulated animals. In the different species, 

 the second layer of this apparatus is formed of two pairs of pieces placed laterally 

 under the basilar piece, of a triangular form, adhering to each other, and with 

 fibres running in different directions. These pieces widen from their base to their 

 summit ; they form a sort of roof over the animals, or rather they form of them- 

 selves a true shell. The basilar piece, or that of the first layer, is often intimate. 

 ly united to them. These two pairs of pieces correspond to the costal and poler- 

 gal pieces of the articulate animals. The third or inferior layer, which is commonly 

 lateral, is situated beneath the pieces of the second layer, and is applied directly 

 upon the body of the animal. It is composed of two pairs of transverse pieces, 

 placed the one above the other. These pieces are highly developed in the Chiton 

 albus, in which they occupy the whole extent of the under part of the sheU. In 

 a New Holland species, they acquire the maximum of their development, extend- 

 ing beyond the edges of the shell, and seeming to form a new shell of themselves. 

 In other cases they are very small, rudimentary, or deduced to mere spines or 

 bony dots. They are then shifted outwards upon the edges of the second layer, 

 which is directly applied to the body of the animal. At other times, these pieces 

 do not appear to exist at all, as in Ch. biradiatus, aculeatus, &c. They are 

 commonly nacreous, a fact which it is of importance carefully to note. Of these 

 two pairs the upper corresponds to the arthromeral, and the other to the arthoceral 

 of the articulate animals. The least important of these different pieces in the 

 chitons, is the basilar, which is always placed upon the second layer, maybe de- 

 tatched from it, and may be replaced by it. The pieces of the latter extend over 

 the body of the animal in conchoidal pieces ; while the iymermost pieces are na- 

 creous, and correspond to the nacreous parts of other shells. 



In the patellae the animal differs essentially from the chitons, in having a 

 single solid or vertebral apparatus, convex externally, and concave within. Yet the 

 shell of the patella is formed precisely in the same manner. The first layer, or 

 basilar piece, is superior, and at the top of the shell. The second layer is com- 

 posed of two pairs of pieces. The pieces of the upper pair, corresponding to the 

 costal pieces, surround the basial at the outside, are more or less intimately uni- 

 ted to it, and are always united to each other and to the upper edge of the pieces 

 of the second pair. The pieces of this second pair, which correspond to the 

 polergal pieces, are inferior to those of the first pair, united to them and to each 

 pther, and from the greater part of the external shell. In this manner, the first 

 two layers of the shell of the patella are entirely external. But the third layer is 

 internal, its upper surfece being united to the other, and its under surface direct- 

 ly applied to the body of the animal. This layer is also composed of two pairs 

 of pieces, which form two portions of distinct layers, internal to the shell and 



