LAMELLIBRANCHIATA. 457 



rior side of the shell, and sometimes project as far as the anterior margin. The 

 length of a shell is the distance from the anterior to the posterior side ; the width 

 is measured from the hinge or dorsal side to the base ; the thickness is measured 

 through the center of the two valves. The surface of the shells is generally marked 

 with ribs, radiating from the umbones, or concentric lines marking the growth of 

 the shell from the umbones. A depression, anterior to the beak, is called a lunule, 

 and when a depression exists posterior to the beak, it is called an escutcheon. 

 Many shells have an external hinge ligament behind the umbones; some have a 

 ligament between the umbones. When the valves are connected internally by a 

 single muscle, the contraction of which brings the valves together, they belong to the 

 Order Monomyaria ; if there are two equally developed contracting mussels, they 

 belong to the Dimyaria; or if there are two muscles, one large and functionally act- 

 ive, the other small, they belong to the Heteromyaria. These contracting muscles 

 are called the adductors, and their places of attachment are indicated by scars. The 

 border of the mantle makes an impression, which is called the pallial line, and if 

 there is a sinus in the posterior part of the pallial line, it shows the animal had a re- 

 tractile siphon, which, in burrowing shells, is often of great length. The Class has 

 also been divided into two Orders, based on the presence or absence of a siphon, 

 to wit : Asiphonida, Asiphonata, or Asiphonophora, and Siphonida, Siphonata, or 

 Siphonophora. Each Order is spelled three different ways by different authors. 

 Shells having a siphon are always gaping at the posterior or anterior side or at both. 

 It will be observed from the foregoing, the essential characters upon which Pal- 

 aeozoic shells are classified are the following : Equality or inequality of the valves ; 

 the presence or absence of an external ligament ; the number of muscular scars ; 

 the character of the hinge and its dentition ; the presence or absence of a pallial 

 sinus; the position of the umbones; the radiate or concentric surface markings; 

 whether the valves fit each other or are gaping at one or both ends ; and the pres- 

 ence or absence of a byssal sinus. 



Order Asiphonida. 



Ambonychiidae, Amphicoeliidae, Anodontopsidae, Arcidse, Aviculidao, Aviculo- 

 pectenidse, Cytberodontidse, Modiomorphidae, Mytilidae, Nuculidae, Nyassidte, Ortho- 

 notidae, Ostreidae, P,alaeoconchidae, Pinnidse, Prothyridae, Pteriniidae, Technophoridae, 

 Trigoniidae, Unionidae. 



Order Siphonida. 



Cardiidae Cardiomorphidae, Conocardiidae, Cyprinidae, Eopteriidae, Grammysiidae, 

 Lucinidae, Myacidae, Palaeanatinidae, Pholadellidae, Sanguinolitidae, Solenidae, Spiro- 

 domidae, Tellimctae. 



Family Ambonychiid^k. — Ambonychia, Angellum, Anomalodonta, Bys- 



sopteria. 

 Family Amphkxeliid;e. — Amphiccelia. 

 Family Anodontopsidse. — Anodontopsis, Cycloconcha. 

 Family ARCiDiE. — Carbonarca, Clinopistha, Macrodon, Megalomus, Ptycbo- 



desma. 



