5l8 TRANSACTIONS OF THE WAGNER FREE 



we recede in Geological time is very marked, and we cannot doubt their rela- 

 tionship to the Nuculacea, notwithstanding all changes. The recent abyssal 

 forms often have the gill filaments simple, but in the shallow-water species they 

 are reflected. In recent Pectuncultis the ventricle is more compact about the 

 rectum and the foot recalls that of Nucula, while the adults are not byssiferous. 

 The wide range of variation in those species of Arcacca in which the anatomy 

 has been examined renders generalizations a little unsafe. In general the 

 gills of the abyssal forms are destitute of the chitinous skeleton which supports 

 the filaments in the littoral species, and are more threadlike, and the filaments 

 not always reflected. 



B. Schizodonta. 

 Superfamily PTERIACEA. 



Shells of varied form, frequently alate, with a nacreous or subnacreous 

 inner and prismatic outer layer, the epidermis seldom conspicuous ; areaamphi- 

 detic, ligament variable, usually not parivincular ; gills filibranchiate or reticu- 

 late, usually reflected ; pleural ganglion fused with the cerebral ; mantle lobes 

 free, without siphons; pallial line simple; the anterior adductor smaller, or fre- 

 quently obsolete in the adult, though present in the young; generally byssifer- 

 ous ; hinge schizodont or edentulous. The young sometimes showing a dis- 

 tinct nepionic stage. Marine. 



Owing to the varied significations adopted from time to time for the Lam- 

 arckian Malleacea, I have preferred to use a different term, based on the best- 

 known genus, Avicula Lam. (= Pteria Scopoli), for the present group. 



FAMILY PTERINEIDyE. 



Shell Pteriiform, bialate, dimyarian.the anterior adductor smaller; inequi- 

 valve, very inequilateral; dentition obscure; ligament amphidetic, external, 

 multivincular? the byssus passing through a notch in the smaller valve. 



Lower Silurian to Devonian. 



Ex. Rhombopteria, Pterinea, Actinopteria, Ptychopteria, ? Leptodesma. 



In Pterinea and its allies we have the first indications of divergence to 

 what ultimately became taxodont and schizodont dentition. From this assem- 

 blage, as indicated by Jackson, a large proportion of the Prionodesmacea have 

 diverged in various directions. It is probable that from this source the fili- 

 branchiate Taxodonts have sprung, rather than directly from the foliobranchs. 



FAMILY AMBONYCHIID/E. 



Shell mytiliform, with no anterior wing, the anterior adductor obsolete ; 

 equivalve, very inequilateral ; dentition obsolete or schizodont ; ligament ex- 

 ternal, multivincular ? byssus passing through a narrow gape between the 

 valves which are otherwise closed. 



