226 Natural' Historical Collections. 



cise shell already described and predominant in the pholades. All its pieces may 

 be free and detached. Thus, the basial is single, the costal pieces and polergal 

 pieces, which are very distinct in the Teredo clava, form the large valves ; the 

 arthromeral pieces are raised outwards, as in the Fhoias costata ; and the arthro- 

 ceral pieces, as in the pholades, constitute two small internal valves. But the 

 long tube, which envelopes the animal, cannot be rigorously determined by itself 

 in the teredines. We must therefore have recourse to other genera, and the pho- 

 lades assist in clearing up this difficulty. In fact, several pholades are double- 

 shelled, as will readily be perceived. Under the large valves of these species,, 

 there occurs a new solid apparatus, which has been remarked and figured by se- 

 veral authors. It is composed as follows : \st, There is a basial piece situated 

 under the shell opposite to the first basial, between the lower edges of the valves. 

 It is elongated, and may not be obvious. Then there are three pairs of solid 

 pieces, often very small, and of different forms, which follow or accompany this 

 basial piece. They represent the costal, arthromeral, and arthroceral pieces. In 

 lact, the two pieces, which seem to constitute nearly the whole of this shell, are 

 more developed and nearer the oral orifice. They are the two anterior pieces of 

 this shell, the two polergal pieces in a state of great extension. Before this ap- 

 paratus can be rightly understood, all the species must be examined, for in some 

 pholades the pieces are not all evident, and in others they may not all exist. The 

 tube of the teredo is formed of the polergal pieces of this second shell. The Sep- 

 tariae and Fistulanse are similarly constituted. 



This theory of the shell of the bivalve mollusca renders that of the univalves 

 perfectly easy, although without it we could not possibly obtain any satisfac- 

 tory result. Thus, the Calyptrea form perfect modes of the composition of the 

 univalve shell as above explained. In the clochettes and cabochons, the ba- 

 sial, which is superior and very distinct in several species, at length disappears. 

 The external pieces, or those of the second layer are disposed circularly around 

 this basial piece ; while the pieces of the third layer, which are distinct and al- 

 ways nacreous, are remarked within. The arthroceral piece of several cabochons 

 detaches itself and remains free. It begins to twist, and then unites with the ar- 

 thromeral, which is near it, and continues to twist in a spiral manner, thus form, 

 ing th^ axis or columella. The external pieces are moulded upon this spire. 

 In the fissureUa;, the basial piece is wanting, and there results the hole or aper- 

 ture from which the genus takes its name. We have thus all the elements of a 

 valve of the bivalve mollusca. 



Most of the other shells called univalve are easily explained upon the same 

 principles, if it be admitted that their operculum is a rudimentary valve, a fact 

 which has already been advanced by several authors. These shells, however, 

 ought not to bear a name which is only applicable to the Calyptreas. There are 

 indeed among the univalves several genera and many species which have no oper- 

 culum, but which cannot be conveniently placed in another family or tribe. 



The anatifcB have their organs contained in a single solid apparatus, which pre- 

 sents the following arrangement : — 1 . The basial piece, which is simple, atte- 

 nuated, keeled, and placed longitudinally along the back. 2. The arthromeral 

 pieces, situated at the upper part, triangular and in the form of valves. They 

 open towards the summit for the mouth and branchiae of the animal ; and some- 

 times form the predominant pieces. 3. The arthroceral pieces situated under the 

 arthromeral form the pieces which are generally the largest. They resemble two 

 valves composed of a single element. These two pairs of pieces are always smooth, 

 and resemble each other. Several anatifse have no other pieces ; but others have 

 the costral and polergal pieces, which form the four solid laminse observed at the 

 lower part in many species. They are often rudimentary. The otions have only 

 the basial piece and the two arthroceral. The latter are not distinguishable in 

 the cineras. 



The author then concludes with some general considerations. 1. If the results 

 above stated are real, it is obvious that the shell of the molluscous animal confirms 

 the principle already established, that five pairs of elements enter into its compo- 



