BIVALVE SHELLS. 59 



by Area and Sponcljlus. The central teeth, those immediately 

 beneath the umbo, are called hinge (or cardinal) teeth (ii, 31 ) ; 

 those on each side are lateral teeth (ibid.\ Sometimes lateral 

 teeth are developed, and not cardinal teeth (Kellia) : more fre- 

 quently the hinge-teeth alone are present ( Alasmodon ). In young 

 shells the teeth are sharp and well defined ; in aged specimens 

 they are often thickened, or even obliterated b^^ irregular growth 

 (Hippopodium) or the encroachment of the hinge-line (Pectun- 

 culus ;. The dentition of bivalve shells may be stated thus : — 

 cardinal teeth, 2*3 or § — meaning 2 in the right valve, 3 in the 

 left; lateral teeth 1 — 1, 2 — 2, or 1 anterior and 1 posterior, in 

 the right valve, 2 anterior and 2 posterior lateral teeth in the left 

 valve. Many of the fixed and boring shells are edentulous. 



The muscular impressions axQ those of the adductors, the foot 

 and byssus, the siphons, and the mantle (^ii, 30, 31). 



Fossil bivalves are of constant occurrence in all sedimentary 

 rocks ; the}'' are somewhat rare in the older formations, but 

 increase steadilj'^ in number and variety through the secondary 

 and tertiary strata, and attain a maximum of development in 

 existing seas. 



Some families, like the C3'prinid8e and Lucinidfe are more 

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

 family (the Hippuritidse), have become extinct. The deter- 

 mination of the aflflnities 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 frequentl}^ 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 difficult}^ arises from the frequent destruction of the 

 nacreous or lamellar portion of the fossil bivalves, whilst the 

 cellular layers remain. The Aviculidge of 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 the 

 shells could not be detei'mined from such specimens. Our 

 knowledge of the extinct Radiolite is derived from natural 

 moulds of the Interior, formed before the dissolution of the 

 inner layer of shell, or from specimens in which this layer is 

 replaced by spar. 



The necessities of geologists have compelled them to pay very 

 minute attention to the markings in the interior of ihells, to 

 their microscopic texture, and every other available source of 

 comparison and distinction, it must not, however, be expected 



