404 MANUAL OF THE MOLLUSCA. 
less shell than the mantle, and their impressions deepen with 
Age. 
The pallial line (Fig. 214, m)is produced by the muscular 
fibres of the mantle-margin; it-is broken up into irregular 
spots in the monomyary bivalves, and in Saxicava and Panopea 
Norvegica. 
The siphonal impression, or pallial sinus (Fig. 14, p. 20), only 
exists in those shells which have retractile siphons; its depth 
is an index to their length. The large combined siphons of 
Mya (Fig. 207) are much longer than the shell; and those of 
some Tellinide three or four times its leneth, yet they are com- 
pletely retractile. The small siphons of Cyclas and Dreissena 
cause no inflection of the pallial line. The form of the sinus 
is characteristic of genera and species. 
In the umbonal area (within the pallial line) there are some- 
times furrows produced by the viscera, which may be distin- 
guished from the muscular markings by absence of polish and 
outline. (See Lucina, Pl. XIX., Fig. 6.) 
Fossil bivalves are of constant occurrence in all sedimentary 
rocks; they are somewhat rare in the older formations, but 
increase steadily in number and variety through the secondary 
and tertiary strata, and attain a maximum of development in 
existing seas. 
Some families, like the Cyprinide and Lucinide are more 
abundant fossil than recent; whilst many genera, and one whole 
family (the Wippuritide), have become extinct. The determi- 
nation of the affinities of fossil bivalves is often exceedingly 
difficult, owing to the conditions under which they occur. 
Sometimes they are found in pairs, filled up with hard stone; 
and frequently as casts, or moulds of the interior, giving no 
trace of the hinge, and very obscure indications of the muscular 
markings. Casts of single valves are more instructive, as they 
afford impressions of the hinge.* 
Another difficulty arises from the frequent destruction of the 
nacreous or lamellar portion of the fossil bivalves, whilst the 
cellular layers remain. The Aviculide of the chalk haye entirely 
lost their pearly interiors; the Spondyli, Chamas, and Radiolites 
are in the same condition, their inner layers are gone and no 
vacancy left, the whole interior being filled with chalk. As it 
is the inner layer alone which forms the hinge, and alone 
receiyes the impressions of the soft parts, the true characters of 
* These impressions may be conveniently moulded with gutta-percha. M. Agassiz 
published a set of plaster-casts of the interiors of the genera of recent shells, which 
inay be seen in the Brit. Museum. [A/emoire sur les moules des Mollusques, vivans et 
fussiles, par L. Agassiz, Mem. Soc. Sc. Nat. Neuchatel, t. 2.] 
