These scars mark the attachment of the adductor muscles, for drawing the valves together, and the shells with only one impression on each valve are called _monomyary_, those with two impressions _dimyary_. The discovery of monomyary shells indicates with tolerable certainty the marine character of the deposit in which they are found, though their absence cannot be taken as proof of freshwater origin. The beaks or umbones of the bivalves are often corroded in freshwater deposits, as may be seen by examining shells of the common freshwater mussel. "All univalve shells of land and freshwater species, with the exception of _Melanopsis_ and _Achatina_, which has a slight indentation, have entire mouths; and this circumstance may often serve as a convenient rule for distinguishing freshwater from marine strata; since if any univalves occur of which the mouths are not entire, we may presume that the formation is marine[33]." [Footnote 33: Lyell's _Students' Elements of Geology_, Second Edition (1874), Chap. III. A good account of the differences between freshwater and marine organisms, from which some of the facts here cited are extracted, will be there found.] [Illustration: Fig. 12. _A._ Holostomatous shell. _B._ Siphonostomatous shell.] In Fig. 12 _A_ shows a freshwater shell (_Vivipara_) with entire mouth, whilst _B_ exhibits the shell of a marine gastropod (_Pleurotoma_) with a notched mouth. The entire-mouthed shells are called _holostomatous_ whilst those which are notched, the notch being often prolonged into a canal, are termed _siphonostomatous_. Many groups of invertebrates are seldom or never found in fresh water. Of exclusively or nearly exclusively marine creatures we may name the foraminifera, radiolaria, sponges with a hard framework, most hydrozoa which secrete hard parts, corals, echinoderms, cirripedes, king-crabs, locust-shrimps, most polyzoa, brachiopods, pteropods, heteropods, and cephalopods. Of extinct groups, the graptolites and trilobites seem to have been entirely confined to the sea.