468 MOLLUSCA phylum vi 



Family 9. Lyonsiidae Dali. 



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 Kecent. 



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

 ture. Tertiary and Eecent. 



Entodesma Pliilippi, Kecent. Actinomya Mayer. Eocene ; North America, 



Family 10. Lyonsiellidae Dali. 



Shell nearly equivalve, sub-nacfeous, 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 Dali ; Lyonsiella Sars. Chiefly Recent. 



Superfamily 2. ENSIPHONACEA DaU. 



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 f ringe of small calcareous tubules. 



Family 11. Clavagellidae d'Orbigny (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 narroio 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. 



Glavagella Lam. (Bryopa Gray ; Stirpulina Hol.) (Fig. 746). One 

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

 Cretaceous to Recent. 



Brechites Guett. {Äspergillum Lam.). Both valves merged in the 

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

 „ ^,. and Recent. 



Fig. 746. 

 Glavagella 



caüiati Desh. Superfaiüily 3. POROMYACEA DaU. 



Eocene; Grig- 



Desiiayes)* ^ 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 Dali. 



Shell sub-equivalve, nacreous and cellulo-crystalline, externally granulöse; 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, 

 internal, with a strong lithodesma ventrally ; area obscure or absent ; a depressed false 



