534 TRANSACTIONS OF THE WAGNER FREE 



The soft parts differ, but the shell of Chamostrea looks much like a sessile 

 Halicardia. 



Superfamily ENSIPHONACEA. 



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

 may involve one or both of the valves and which is usually furnished with a 

 perforated anterior disk surrounded by a more or less complete fringe of small 

 calcareous tubules. 



The formation of the disk in this group is wrapped in mystery, as, in those 

 specimens which have been examined, neither the mantle nor the foot show 

 any papillae or other projections through whose agency the tubules might be 

 formed. In specimens which are enclosed in hard rock the disk is sometimes 

 imperfectly formed. 



FAMILY CLAVAGELLID^E. 



Ventricle embracing the rectum ; gills reticulate, with the outer direct 

 lamina reduced, united behind and prolonged into the siphonal conduit ; foot 

 digitiform, grooved, rudimentary ; the mantle with small pedal and opisthopo- 

 dial foramina ; siphons united nearly to their tips, papillose ; the branchial with- 

 out a curtain valve ; monoecious, marine. 



Shell degenerate, extremely specialized for a burrowing life ; valves nacre- 

 ous, 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, the pallial line sinuate ; the tube frequently encrusted with extra- 

 neous material. 



Cretaceous to recent fauna. 

 Ex. Clavagella, Brechites, Foegia, Humphreyla. 



In Clavagella one of the valves is not attached to the tube, and the ad- 

 ductor muscles persist. In Brechites both valves are merged in the tube, the 

 anterior adductor degenerates, and the posterior disappears entirely. 



Superfamily POROMYACEA. 



Anomalodesmacea having modified foliobranch or lamellar gills, slightly 

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

 when fully developed, attached to one another by interlocking giant cilia, dis- 

 posed in bands or patches and by occasional connective fibres which are not 

 of a vascular nature ; often with a special development or specialization of the 

 branchiosiphonal septum; the valves free, without a calcareous tube external 

 to them ; the mantle lobes united, with siphons and a pedal, but no opisthopo- 

 dial foramen ; the resilium reinforced below by a lithodesma. 



This group is strongly contrasted with the typical Anatinacea by the sim- 

 ple laminar gills, when present, and by the tendency to obsolescence of the 



