576 MOLLUSCA 



partitions, the ascending limb of the outer leaflet being united with the 

 mantle; genital glands have independent openings to the outside; two 

 adductor muscles usually present; mantle edges more or less fused and 

 the siphons present (Fig. 888), either in form of closed tubes or not; 

 foot usually large, with byssus little or not at all developed: about 27 

 families, grouped in 7 suborders, including most pelecypods. 

 Key to the suborders of Eulamellibranchiata : 



Oj Mantle often below, shell usually with no pallial sinus. . . .1. SUBMYTILACEA 

 a, Shell usually thin, with pallial sinus (Fig. 889, 10) ; siphons usually 



present. 

 &! Pallial sinus prominent. 



G! Pallial sinus extending far forwards 2. TELLINACEA 



c a Pallial sinus not so large. 

 di Animals do not bore in solid objects. 



e t Shell thick and solid and more or less round 3. VENERACEA 



e 2 Shell rather thin and usually elongate 5. MYACEA 



d 3 Animals bore in clay, wood, or rock G. PHOLADACEA 



6, A slight pallial sinus or none present. 



<?! Shell thick, usually ribbed 4. CARDIACEA 



c a Shell thin 7. ANATINACEA 



SUBORDER 1. SUBMYTILACEA. 



Mantle edges open below; closed siphons and pallial sinus usually 

 absent; 2 adductor muscles; cardinal and lateral teeth (Fig. 889) well 

 developed: about 16 families, in fresh and salt water. 



Key to the families of Submytilacea here described: 



a t Animals marine. 

 &! Shells not minute, although in some cases small. 



Cj_ Shells radially grooved 4 1. CABDITIDAE 



c 3 Shells not radially grooved. 

 dj, Umbo not at forward end. 

 &L Lunule present (Fig. 889, 4). 

 /! Adductor muscle impressions nearly equal in size. 



ff t Shell mostly more than 20 mm. long 2. ASTARTIDAE 



g 2 Shell mostly less than 8 mm. long 3. CRASSATELLIDAE 



/ 2 Anterior muscle impression much the longer 7. LUCINIDAE 



1 Lunule absent 4. CYPRINIDAE 



d 3 Umbo at forward end 6. DREISSENSIIDAE 



6 a Shells minute. 



c a Ligament external 8. KELLYELLIDAE 



c a Ligament internal 9. ERYCINIDAE 



a a Animals in fresh water. 



&! Shell large 5. UNIONIDAE 



6 3 Shell minute 10. CYRENIDAE 



FAMILY 1. CAEDITIDAE. 



Shell heart-shaped or ovate, with radiating ribs or striations, hinge 

 thick, with 1 or 2 cardinal and 1 or 2 lateral teeth; foot with byssus; no 

 closed siphon: about 10 genera, all marine. 



