Natural- Historical Collections. 225 



most shells, have the following arrangement : — The first pair, the arthromeral, 

 constitute the prominences, which are designated by the name of naies, or umboiies. 

 They arise from the posterior edge of the basial piece, proceed backwards and 

 upwards, and may even extend to the middle of the large valves, of which they 

 then become integrant parts. These pieces are smoother than the others. They 

 soon enter into the interior of the shell; but they may be cast entirely out- 

 wards, and, in that case, form only an external and recurved lamina, as in 

 Ph. costata, dactylus, and cristata. The second pair of pieces of this layer, the 

 arthroceral, are sdtogether internal. They are often free and detached from the 

 neighbouring pieces, in which case they form the internal valves. When they 

 are adherent and intimately attached by their upper surface, they form the cuil- 

 leron or spoon-like process of each valve. Their inner surface is always nacreous, 

 as is that of the first pair, from their entrance into the inside of the shell. 



In short, the pholades present a basial piece, often free, and occasionally com - 

 posed of two distinct pieces. The costal and polergal pieces commonly form the 

 greater part, and often almost the whole of the large or outer valves ; and then 

 their fibres may have opposite directions. The arthromeral, which are common- 

 ly shapeless in the other bivalve moUusca, may here constitute a jjair of entirely 

 distinct and even external pieces, which form the nates, and tend to enter into 

 the inside of the shell. They generally have a nacreous appearance. The ar- 

 throceral pieces, or internal valves of authors, are always nacreous, and some- 

 times free ; but they are also united to the outer valves, and then form the cuil- 

 lerons of authors. 



If the calcareous pieces above described in the chitons, patellae, and pholades, 

 correspond to the vertebral elements of other animals, we have thus the iive ele- 

 mentary pairs of pieces which enter into the composition of the vertebra, or of a 

 vertebral segment ; and thus the true theory of the shell is evolved. 



All shells in existence, even the internal solid apparatus of the cuttle-fish, are 

 formed upon a similar model, and may be referred to the above-mentioned pieces. 

 Thus, the solens are merely pliolades, whose internal pieces are narrowed, while 

 the external pieces have undergone a very great elongation. In these animals, 

 the basial piece is stiU external, but almost confined to the hinge. The costal 

 and polergal pieces constitute the two plates of the valves, of which the costal 

 form the antero-inferior part, and the polergal the whole upper and antero-supe- 

 rior part. The fibres of these different pieces run in different directions, by 

 which they are easily distinguished. The internal valve of the pholades (the 

 arthroceral pieces,) here constitutes a small spine situated towards the base of the 

 shell ; but the arthromeral pieces are represented by the turned back piece to- 

 wards the base of this small spine, near the posterior edge of each valve. 



Without this theory, the author thinks there are no means of explaining the 

 structure of the bivalve mollusca, which are now so easily understood. The dif- 

 ferent pieces are indeed very diflTerent in their development in the different tribes, 

 but they can easily be distinguished with a little attention ; and even in the most 

 embarrassing genera, analogy leads to a sure discrimination. 



In the series of bivalve mollusca, the basial piece is generally confined in the 

 hinge ; the costal and polergal pieces at the exterior form the external portion or 

 lamina of the valves ; the arthroceral pieces, at the interior, constitute the nacreous 

 plate, which is named the cuilleron ; and the arthromeral pieces, which are are 

 also nacreous and internal, commonly form the greater part of the articulation, 

 and extend more or less into the inside of the shell. In their greatest develop- 

 ment they surround the cuilleron, and give rise to the pallial impression, which 

 indicates their true points of separation. In like manner, the polergal pieces 

 may turn round upon the costal pieces to complete the external lamina of the 

 Talves. 



Among the mollusca, the author finds several genera which really have two 

 shells. The genus Teredo, for example, has an internal shell, and an external 

 «>hell. The internal shell, or that of all the merely bivalve mollusca, is the pre- 

 VOL. II. 2p 



