INSTITUTE OF SCIENCE, PHILADELPHIA. 547 



external, often obsolete ; resilium usually internal, subumbonal or oblique ; 

 hinge-plate narrow, channelled to receive the resilium ; hinge variable, normal- 

 ly consisting of one or two radiating cardinals and a pair of lateral lamina; in 

 each valve, the anterior laminae often absent, and the posterior frequently close- 

 ly adjacent to the resilium, simulating cardinals. 

 Tertiary and recent faunas. 



Ex. Erycina (Fischer), Kellia, Mysella, Pythina, Cyamium, Lasea, Lepton, Erycinella, 

 I.nletina, Spatriodon, Fabel/a, Sportella, Entova/va. 



I have preferred the name Leptonidte on account of the uncertainty at- 

 taching to the name Erycina, which has been so variously used. In this group 

 the beaks frequently exhibit the prodissoconch conspicuously. The genus 

 Lcpton is generally commensal, and sometimes attaches itself by a byssus to 

 living crustaceans, or lives in their burrows. Kellia nestles in vacant Pholad 

 burrows, and is often greatly distorted by growth in confined situations; Ento- 

 I'o/i-a is parasitic in the oesophagus of Holothurians, and its byssal apparatus 

 has been modified into a kind of sucker. 



FAMILY GALEOMMID^. 



Gills normal, subequal, united behind; palpi and foot as in Lef>tonid<z ; 

 mantle edges closely united ventrally, except for the pedal opening, reflected 

 permanently over a large part of the valves, papilliferous ; the adductors nor- 

 mal, subequal ; monoscious ? marine. 



Shell without a perceptible epidermis; valves equal, free, widely gaping 

 ventrally, smooth or variably sculptured ; adductor scars distant, oval ; pallial 

 line simple; ligament usually obsolete; resilium internal, subumbonal or 

 oblique, attached to an excavated chondrophore in each valve; hinge-plate 

 hardly developed ; laterals obscure or absent ; one or two cardinal teeth in 

 each valve or none. 



Tertiary and recent faunas. 

 Ex. Scin/i/la, Galeotiuna, Hindsiel/a, Ephippodonla, f Thyreopsis, f Libratula. 



FAMILY CHLAMYDOCONCHID^E. 



Gills, palpi and foot as in Galeommidce, mantle thick and almost cartilagi- 

 nous, enveloping the whole animal, superficially papillose with a very anterior 

 pedal foramen ; incurrent and excurrent orifices dorsally situated, the posterior 

 ventral surface continuous; adductor muscles wholly absent; monoecious? 

 marine. 



Shell cellulo-crystalline, without an epidermis, comprising the prodisso- 

 conch with narrow, long laminar accretions, very small; valves wholly internal, 

 not connected, contained in laterodorsal separate capsules, without hinge or 

 hinge-plate, not attached to muscles or ligament ; ligament absent, resilium 

 separately encapsuled between the obsolete valves, without function. 



Recent. 

 Ex. Chlamydoconcha. 



