institute of science, philadelphia. 527 



Family AN0MIID.<E. 



Shell variable, when sessile irregular and inequivalve, byssiferous when 

 young ; in most of the genera the byssus becomes modified to a calcified or 

 horny plug passing through a foramen in the attached valve and attached to 

 other bodies, a condition which may be permanent or transient ; area small, 

 amphidetic ; ligament amphidetic, more or less internal, supplemented by an 

 internal resilium, for which the crura serve as chondrophores ; ali- or multivin- 

 cular ; hinge usually edentulous, rarely rugose with amorphous interlocking 

 rugosities ; posterior adductor small, subcentral, in the sessile forms reinforced 

 by the pedo-byssal muscles, which are modified for service as adductors. 



Devonian to recent fauna. 

 Ex. Limanomia, Anomia, Placunanomia, Pododesmus , Enigma, Carolia, Monia, Ephippi- 



um, Placenta, Placunema, Hypotrema, Placunopsis, etc. 



The inter-relations between the sessile and free forms are so gradual and 

 close that they cannot be separated as families, though extremes are very un- 

 like. In the former the auricular crura are obscure, except in Placunaiiomia 

 and Carolia ; in the free forms they are well developed, the internal ligament 

 uniting the outer edges in one valve with the inner edges in the other, while 

 the resilium lies between the crura, medially, Placenta seems always free and 

 byssiferous, but all the others, including some species usually referred to Pla- 

 centa, have at some time a byssal foramen and are probably then sessile. 

 Hypotrema has multivincular ligamentary grooves on the area ; Plaainanomia 

 has a singularly amorphous rugose hinge margin, with the rugosities inter- 

 locking ; all the others seem to have a flat, striated, narrow area with an eden- 

 tulous hinge. Ainigma is modified for life on leaves which dip into tidal cur- 

 rents. 



D. Dysodonta. 



Superfamily MYTILACEA. 



Shell anisomyarian, usually equivalve, not alate or notched for a byssus; 

 edentulous or dysodont ; shell substance subnacreous, rarely more or less 

 prismatic, with a conspicuous epidermis ; area amphidetic or obscure ; ligament 

 parivincular, usually opisthodetic, usually external ; ventricle embracing the 

 rectum ; gills usually filibranchiate, usually with direct and reflected limbs ; 

 ganglia normal, mantle lobes without ocelli, more or less free, usually with the 

 anal siphon complete and the branchial incomplete ; otocyst impervious ; foot 

 small, digitiform, grooved, byssiferous ; anal end of rectum sessile ; monoeci- 

 ous, usually marine. 



Family MODIOLOPSID^. 



Shell modioliform, usually equivalve, free, thin, with subequal adductor 

 scars ; ligament deep-seated ; hinge edentulous or dysodont ; sometimes bys- 

 siferous. 



Silurian. 

 Ex. Modiolodon, Modiolopsis, Modiomorpha, Goniophora, Eurymya, Arisiella, Prolobella, 



