Introduction to Animal Morphology. 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. Modiolarcidae 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 vcntrally, with hinge teeth : this comprehends 



ib-ordors: ist Arcaceae mantle edge free, rarely si- 

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

 foot large ; its families are : Arcidae uniforal ; hinge teeth 

 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; 



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

 r leaping : including one family mostly of fossil forms, 

 ex. Trigonia. Sub-order 3. Unionacece (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 < \- 

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

 families are : Lucinidce cquivalve ; a branchial siphon ; 

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



.odon), or band-like (Lucina). Astartidx uniforal ; 

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



:la), or external ; foot rarely 

 with " .lilirardia; ; shell concentrically (Astarte) 



'.ially riM iita). Galmmmido.' i 



mantle . ieriini; tl. 



U 11. Leptnii- .<>nal, Ussiferous ; f.M.t 



ligul./ -ring: mantle with (Lcplon). or without ten- 



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



