Introduction to Animal MorpJiology. 269 



family includes Mussels (Mytilus) without (and Crenella, Mo- 

 diola and Lithodomus with) false siphons. Dreissensidae — 

 umbones terminal, each with a shelf-like septum, and a trifo- 

 ral mantle. Modiolarcidse — triforal; with no septum in the 

 non-terminal umbo. 



Order 3. Isomya. — adductor scars two, equal or sub- 

 equal on each valve : this order contains two sections : — 

 § I. Integropalliatae — pallial sinus none ; shell regular, 

 rarely gaping ventrally, with hinge teeth : this comprehends 

 five sub-orders : ist Arcaceae — mantle edge free, rarely si- 

 phonate (Ledidas) ; hinge teeth, many; cleft, symmetrical; 

 foot large ; its families are: — Arcidse — uniforal ; hinge te^\;h 

 in a straight line ; gills sub-pinnate ; oral tentacles as 

 processes of the gills ; byssus present (Area) or none (Pectun- 

 culus). Nuculidae — hinge teeth serrate ; shell anteriorly elon- 

 gate ; ligament internal. Ledidae — labial tentacles long, 

 convolute ; siphons retractile ; ligament internal (Leda) or 

 external (Malletia). Sub-order 2. Trigoniaceae — uniforal; 

 hinge teeth v-like, diverging ; mantle-edge with eyes ; foot 

 bent, for leaping ; including one family mostly of fossil forms, 

 ex. Trigonia. Sub-order 3. Unionaceae (Naiades) — uniforal 

 (siphonate in Spatha, &c.) ; one family of freshwater, olive 

 shells with thick epidermis, nacreous within ; foot not byssi- 

 ferous (except Byssanodonta) ; embryos with at first only 

 one adductor ; cardinal teeth present, with (Unio) or without 

 lateral teeth (Baphia), or absent, with (Barbala) or without 

 laterals (Anodonta). Sub-order 4. Lucinaceae — biforal (ex- 

 cept Astartidae) ; anal opening simple ; foot cylindrical. The 

 families are : Lucinidae — equivalve ; a branchial siphon ; 

 gills one pair ; anterior adductor scar round (Corbis), double, 

 (Cryptodon), or band-like (Lucina). Astartidae — uniforal ; 

 gills two pair, with bearded edge ; lateral teeth one or none ; 

 ligament in a groove (Crassatella), or external ; foot rarely 

 with a byssus (Mytilicardia) ; shell concentrically (Astarte) 

 or radially ribbed (Cardita). Galeommidae — byssiferous ; an 

 anal siphon ; gills two pair ; mantle edges projecting through 

 the gaping shell. Leptonidae — siphonal, byssiferous ; foot 

 ligulate or tapering ; mantle with (Lepton), or without ten- 

 tacles ; ligament internal (Montacuta) or external (Lasea, &c.) ; 



