270 Introduction to Animal Morphology. 



a long anterior mantle tube, open (Lasea), or closed be- 

 low (Kellia). Ungulinidae foot hollow, worm-like, with a 

 terminal erectile gland ; four foliaceous oral palps. Sub- 

 order 5. Chamaceae siphons short, separate; foot small or 

 rudimental; shells unequal ; inner gill pair small. The fami- 

 lies are : Tridacnidce muscle-scars united ; anus with tubu- 

 lar valve ; outer gills unilamellar; shell thick, with (Tridacna) 

 or without a byssus (Hippopus). Chamidae shells adherent, 

 irregular, inequivalve, umbo spiral ; two outer gill plates 

 united behind ; anal valve simple. Sub-order 6. Cyprinaceoe 

 often trifural ; gills unequal ; foot large. Families : Cypri- 

 nidif foot byssilVrous; umbo spiral in Isocardia ; epidermis 

 thick; mantle open in front and below ; siphons wide, short; 

 teeth 2-3 cardinal and i lateral. Cyrenidoe mantle open in 

 front; margins simple ; epidermis thick ; hinge teeth 3.3 or 

 2.2 ; siphons diverging (C'yclas), or united (Pisidium). 

 diida.- Coi -kles, peK-.-ypodal ; siphons short, sessile; shell 

 equivalve, radially ribbed (Cardium), or smooth (Laevicardium, 

 &c. 



2. Sinupalliata pallial line with a deep sinus, mostly 

 pleuroconchal. This includes four sub-orders. Sub-order 6. 

 Veneraceae siphons moderate ; mantle wide, open in front; 

 foot large, pointed ; gills four. The families are : Veneridae 

 foot long, slemler; siphons united at base or for the whole 

 length (l)osinia) ; hinge teeth 2-3, with often a tooth under 

 the lunula, but no other lateral tooth ; a byssus in Tapes, not 

 in Venus. JVtricolidx foot with a byssal groove; siphons 

 unequal, separate to their base ; shells gaping posteriorly. 

 Sub-order 7. Tellinaceae siphons long, fully separate; mantle 

 wide open in front ; outer gill lamellar, often rudimental ; foot 

 large. Families : Paphiidae hinge cartilage present ; si- 

 phons thick, separate from the base ; foot tongue-like ; shell 

 close; pallial sinus small. Scrobiculariidae hinge cartilage 

 present : siphons long, unequal ; shell gaping behind ; foot 

 pointed, not high. Cumingia bores in coral, &c. Tellinidce 

 hinge cartilage none ; ligament strong, external ; gills un- 

 equal ; hinge teeth one or two ; lateral teeth absent (Psam- 

 mobia), or present (Tellina). Donacidae hinge cartilage 

 none ; shell wedge-shaped ; siphons shorter. Sub-order 8. 



