INSTITUTE OF SCIENCE, PHILADELPHIA. 539 



tie free ventrally, not produced into siphons ; the anal orifice complete ; the 

 branchial papillose, functional, but incomplete below; dioecious, marine. 



Shell substance cellule-crystalline, with a pronounced epidermis ; shell 

 rounded or subtriangular, usually with concentric or not radial sculpture ; 

 valves equal or subequal, free, closed; area distinct; ligament and resilium ex- 

 ternal, parivincular, opisthodetic ; beaks prosoccelous ; adductor scars sub- 

 equal, with a distinct anterior pedal scar ; pallial line simple; hinge-plate dis- 

 tinct, hinge with anterior and posterior lamina: and their respective sockets, 

 variably disposed with respect to the valves, and usually more or less obsolete ; 

 cardinal teeth not bifid at the summit, the full cardinal formula ro°oio . but the 

 terminal teeth frequently obsolete. 



Triassic to the recent fauna. 



Ex. Aslarle, Pachylypus. Piesiastarte, Goodalliopsis, Crassatellina, Blicromeris, Eriphyla, 

 Precoma, ? Plychomya, f Plio?ienia, Opis, Seebachia, Woodia. 



A number of genera have been associated with Astarte which probably be- 

 long elsewhere. A very careful scrutiny is often necessary to determine the true 

 tooth-formula, as the armature of the hinge is frequently more or less obsolete. 

 The teeth are frequently marked with strong transverse striation in the line of 

 motion. 



Family CRASSATELLITID^. 



Anatomy as in AstartidcR (the anal siphonal orifice sometimes incom- 

 plete ?) ; marine. 



Shell as in Astartidce, but the valves always somewhat unequal, and 

 usually more or less rostrate, the beaks compressed, erect or opisthocoelous ; 

 ligament internal, more or less obsolete ; resilium large, wholly internal, 

 attached at each end to a chondrophoric pit in the hinge-plate behind the 

 cardinal teeth ; lateral laminae and sockets usually alternated in the valves, 

 the hinge plate heavy, flat ; the posterior cardinal in the right valve very small 

 or obsolete, with no distinct socket in the opposite valve ; full cardinal formula 



li 10 10 1 



Lower Cretaceous to recent fauna. 



Ex. {Crassaiella=) Crassalelliies, Pseuderiphyla, Scainbulaf, Anthonyia. 



The earlier forms of this family have a small resilium close to the nearly 

 marginal ligament. With time, later ones show a gradual descent of these organs, 

 until in some of the more specialized modern forms there is no appreciable 

 ligament remaining and the resilium has become large and deeply immersed. 

 The change is so gradual that no sharp line can be drawn and the various 

 names applied by Conrad to the early phases can only be regarded as of tri- 

 fling value. The parallelism between this group and the Mactridce, in the 

 gradual immersion of the ligament, could hardly be more complete. 



