ORDER ii ANOMALODESMACEA 391 



Coelodon, Carp. Crura >f tin- left valve united l>y a transver.-e lamella. r/ /,/,'. 

 phora, Carp., lias tin- hin^e plate buttressed and a litli<de.-ma. Tertiary and Recent. 



Family 9. Lyonsiidae. DalL 



Shell inequivalve, thin, sub-nacreous, edentulous; ligament obsolete, the resilium 

 internal, uniting the edges of a long, mesial lithodesma to a narrow chondrophoric sub- 

 marginal ridge on each valve; beaks entire, valves nearly closed, pallial sinus distinct; 

 marine. Tertiary and Recent. 



Lyonsia, Turton. Small, thin, posteriorly elongate with delicate radiating sculp- 

 ture. Tertiary and Recent. 



Entodesma, Phillippi. Recent. Actinomya, Mayer. Eoeene ; North America. 



Family 10. Lyonsiellidae. Dall. 



Shell nearly equivalve, sub-nacreous, with a more or less distinct tubercle in front of 

 the resilium on the dorsal margin; ligament obsolete, cartilage internal with a large 

 lithodesma; area obscure or absent; beaks entire; valves almost closed; pallial sinus 

 obsolete. Tertiary and Recent. 



Halicardia, Dall ; Lyonsiella, Sars. Chiefly Recent. 



Superfamily 2. ENSIPHONACEA. Dall. 



Differing from Anatinacea by the formation of a calcareous tube, which may include 

 one or both of the valves, and is usually furnished with a perforated anterior 

 disk surrounded by a more or less complete fringe of small calcareous tubules. 



Family 11. Clavagellidae. D'Obigny (emend.) 



Shell degenerate, extremely specialised for a burrowing life; valves 

 nacreous, free when young ; when adult, one or both merged in a calcareous 

 tube anteriorly discoid and fringed, with a narrow pedal foramen in the 

 middle of the disk ; free valves edentulous, the ligament external, opisthodetic, 

 supported by nymphs ; pallial line sinuate ; tube frequently encrusted with 

 extraneous material ; marine. Cretaceous to Recent. 



Clavagella, Lam. (Bryopa, Gray ; Stirpulina, Hoi.), (Fig. 697). One 

 of the valves not attached to the tube and adductor muscles persistent. 

 Cretaceous to Recent. 



Brechites, Guett. (Aspergillum, Lam.) Both valves merged in the 

 tube,- anterior adductor reduced, and the posterior obsolete. Pliocene 

 and Recent. (.-,,,. 



Superfamily 3. POROMYACEA. Dall. CMBM 



Eocene ; Grig- 



Anomalodesmacea having modified foliobranch or lamellar gills, slightly 

 or not at all reticulated, and frequently degenerate or even absent; valves 

 free, without a calcareous tube external to them ; mantle lobes united, with siphons and a 

 pedal, but no opisthopodial foramen ; the cartilage reinforced below by a lithodesma. 



Family 12. Euciroidae. Dall. 



Shell sub-equivalve, nacreous, and cellulo-crystalline, externally granulose; hinge 

 with a strong tubercle in the right valve before the resilium, and the dorsal margins 

 modified to overlie and underlie each other; ligament obsolete; resilium opisthodetic, 



