22 REPORT — 1844. 



of the branchiae, as in Unio ; and, as in the NayadecB, there is a large foot. 

 When we add to these characters the attachment of the shell by one of ita 

 valves, as in OstracecB and Chamacew, the assemblage becomes very per- 

 plexing. The microscopic structure of the shell here affords, I think, valu- 

 able aid (fig. 52). The prismatic cellular structure here exists in large amount, 

 as in Pinna ; and the interior is nacreous or sub-nacreous. In these respects 

 it entirely differs from the CliamacecE, in which there is not a trace of pris- 

 matic cellular structure, and in which the inner layer has characters which 

 that of Etheria does not possess. 



56. In all the preceding families, the lobes of the mantle are disunited ; 

 and it is very interesting to find hoAv completely the Prismatic Cellular sub- 

 stance is restricted to the group thus constituted. The only approaches to 

 it, which I have met Avith in other Bivalve Mollusca, are among the family 

 Myid(B ; and it is only in the very aberrant genus Pandora, that it shows 

 itself in a truly characteristic form. Of this group I should be disposed 

 to take the MargaritacecB as the typical or central family. From these 

 we might pass off towards the Brachiopoda on the one hand, by the true 

 OstracecB, which conduct us towards the Placunidce. Again, by Avicida and 

 Pinna, we are led towards the MytilacecB. By Etheria we are conducted to 

 the Nayadea, and these lead us towards the Chamacece. The most aberrant 

 family, in respect to the structure of the shell, is that of Pectinida, in Avhich 

 the prismatic cellular structure is entirely absent, whilst there is also an ab- 

 sence of the true nacreous character. Noav although the general structure of 

 the PectinidcB is not usually regarded as widely different from that of the 

 OstracecB, their habits depart most widely from those which prevail in the 

 group ; for while the Oysters ai'e fixed by the adhesion of their shells, and 

 the Margaritacece by a byssus, the Pectens are usually free, and seem to 

 possess more locomotive power, together with a more complete sensory appa- 

 ratus, than any others of the group. It seems to me that, in these respects, 

 they have a relation of analogy with the CardiacecB : and if such a relation 

 exist, it is remarkably borne out by the intimate structure of the shell, which 

 is closely allied in these two families ; as well as by that ribbed surface, which 

 is well known to be characteristic of its exterior, at least in the typical genera 

 of each family. 



List of Illustrations. 



Plate I. — Fig. 1 . Section of Pinna nigrina, parallel to its surface, under 

 a power of 10 diameters; cutting the prismatic cells transversely, and 

 showing the outcrop of the coloured layers (§ 10). — Fig. 2. Section of 

 Pinmi nigrina, perpendicular to its surface, under a power of 50 dia- 

 meters; showing the alternation of coloured and colourless laj'ers (§ 10). 



Plate II Fig. 3. A portion of fig. 1, magnified 185 diameters. — Fig. 4. 



A corresponding portion, after immersion in dilute acid, showing the 

 residual membrane, composed of cells (§ 5, 6). 



Plate III. — Fig. 5. External surface of Pinna marina, shovi'ing nu- 

 merous large dark cells; magnified 185 diameters. — Fig. 6. Section 

 parallel to the surfaces, but through the middle of the thickness of the 

 same layer ; showing a compai'atively small number of dark cells. Mag- 

 nified 185 diameters. — Fig. 7. Internal surface of the same layer; 

 showing the entire absence of the dark cells, and the greatly-increased 

 size of the remainder (§ 5, 14). 



Plate IV. — Fig. 8. Thin (natural) lamina of Pinna ingens, showing the 

 nuclei of the cells. Magnified 300 diameters (§ 6) — Fig. 9. Separate 



