404 MAKUAL OF THE MOLLUSC A. 



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 Punopcea 

 Norvegica. 



The siphonal impression, or pxdlial 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 

 3Iya (Fig. 207) are much longer than the shell ; and those of 

 some Tellinidce three or four times its length, yet they are com- 

 pletely retractile. The small siphons of CycJas and Dreissena 

 cause no inflection of the pallial line. The form of the sinus 

 is characteristic of genera and species. 



In the umhonal 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, PI. 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 CyprinidcB and Lucinidce are more 

 abundant fossil than recent; whilst many genera, and one whole 

 family (the Ilippuritidai), 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 Avicrdidceoi the chalk have 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 

 receives the impressions of the soft parts, the true characters of 



* Those impressions maj' be conveniently moulded •with guftn-perchn. M. Agassiz 

 published a set of plaster-oasts of the interiors of the genera of recent shells, which 

 niiiy be seen in the Brit. Museum. [Mcmoire snr Ics monies des Mollusqties, vivan^pt 

 fussilcs, par L. Agassiz, Mem. Soc. ISc. A^at. Neiichntel,t. 2.] 



