468 



MOLLUSCA 



PHYLUM VI 



Family 9. Lyonsiidae Dall. 



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

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

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

 marine. Tertiary and Recent. 



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

 ture. Tertiary and Recent. 



Entodesma Philippi. Recent. Actinomya Mayer. Eocene ; North America. 



Family 10. Lyonsiellidae Dall. 



SIbell nearly cquivalve, 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 ; fallial sinus 

 obsolete. Tertiary and Recent. 



Halicardia Dall ; Lyonsiella Sars. Chiefly Recent. 



Superfamily 2. ENSIPHONACEA Dall. 



Dijf'ering 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. Olavagellidae d'Orbigny (emend.). 



Shell degenerate, extremely specialised for a burrowing life; valves 

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

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

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

 sup'ported by nymphs ; pallial line sinuate ; tube frequently encrusted with 

 extraneous material ; marine. Cretaceous to Recent. 



Clavagella Lam. {Bryopa Gray ; Stirpulina Hoi.) (Fig. 746). 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. 



Fio. 746. 



Clavagella 

 caillati Uesh. 

 Eocene ; Grig- 

 non. i/o (after 

 Deshayes). 



Anomalodesmacea having modified foliobranch or lamellar gills, slightly 

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

 free, witliout a calcareous tube external to them ; mantle lobes united, until siphons and 

 a pedal, but no opisthopodial foramen ; the cartilage reinforced beloio 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; resilitim opisthodetic, 

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



