ORDER II ANOMALODESMACEA 391 
Coelodon, Carp. Crura of the left valve united by a transverse lamella. Clidio- 
phora, Carp., has the hinge plate buttressed and a lithodesma. Tertiary and Recent. 
Family 9. Lyonsiidae. Dall. 
Shell tnequivalve, thin, sub-nacreous, edentulous; ligament obsolete, the resiliwm 
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, Philippi. Recent. Actinomya, Mayer. Eocene; 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’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 antertorly 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 sinuwate ; tube frequently encrusted with 
extraneous material ; marine. Cretaceous to Recent. 
Clavagella, Lam. (Bryopa, Gray ; Stirpulina, Hol.), (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. caittati, Desh. 
Eocene; Grig- 
Anomalodesmacea having modified foliobranch or lamellar gills, slightly TREE oe 
or not at all reticulated, and frequently degenerate or even absent ; valves 
free, without a calcareous tube external to them ; mantle lobes wnited, with siphons and a 
pedal, but no opisthopodial foramen ; the cartilage reinforced below by a lithodesma. 
Family 12. Huciroidae. 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, 
