INSTITUTE OF SCIENCE, PHILADELPHIA. 515 



this, and with the help of the morphologists, we may reasonably hope in time 

 to arri\-e at a rational and genetic classification. 



Order PRIONODESMACEA 



I (Without Teeth). 

 Superfamily SOLEMYACEA. 



Shell thin, elongate, practically equivalve, low-beaked, with a conspicuous 

 epidermis, substance prismatic ; hinge edentulous ; anal and branchial foramina 

 not separated ; gills foliobranchiate ; pleural ganglion distinct, both adductors 

 present; non-byssiferous. 



Family SOLEMYID^. 



Shell soleniform, gaping, with the anterior end longer and the epidermis 

 conspicuous, exceeding the valves ; area obscure or none ; ligament amphi- 

 detic, parivincular, becoming internal posteriorly; mantle lobes united ven- 

 trally, attached in front to the epidermis and valves by a broad surface, leaving 

 no distinct pallial line ; a single posterior siphonal and anterior pedal foramen 

 in the mantle; palpi united laterally above; adductors subequal, with a thick- 

 ened ray in front of the posterior scar ; ventricle embracing the rectum and 

 giving out a single anterior aorta ; foot elongated, having a pedal gland, but 

 no byssus, with a terminal groove which may be expanded to a stellate disk ; 

 animal dioecious, marine, burrowing. 



Lower Silurian (? Carboniferous) to recent faunas. 

 Ex. Solemya, ? Orthodesma, f Whiteavesia, Janeia. 



The disk of Solemya must be used as a stilt or excavator, though morpho- 

 logically reptary, as in Nucula. The dorsal aspect of the outer row of gill 

 leaflets is accounted for by the vertical position of the animal in the burrow. 

 Those foliobranchs which crawl in a horizontal posture have both rows of 



leaflets directed ventrally. 



11 (Dentiferous). 



A. Taxodonta. 

 Superfamily NUCULACEA. 

 Shell of variable form, closed ventrally, equivalve, with a smooth epidermis ; 

 nacreous or porcellanous with tubuliferous external prismatic layer ; taxodont ; 

 area obscure, or none, when present divided into lunuleand escutcheon ; liga- 

 ment variable, amphidetic ; gills foliobranchiate ; pleural ganglion distinct ; 

 otocysts pervious ; both adductors present and subequal ; foot grooved and 

 reptary, not byssiferous ; marine. 



? Family CTENODONTID^. 



Shell nuculiform, with the teeth in a continuous arched series ; no area ; 

 ligament external, alivincular, without an internal resilium ; pallial line simple. 



Lower Silurian. 

 Ex. Ctenodonta, Cucullella\{? . Fischer), ? NucuHtes. 



