556 TRANSACTIONS OF THE WAGNER FREK 



dorsally. Inner wall of the mantle behind the siphons exhibiting a laminar 

 sense-organ ; aorta with postventricular dilation. 



FAMILY MACTRID^E. 



Gills plicate, normal, the outer reflected lamina usually with a dorsal ex- 

 tension, united behind and to the septum, forming a complete anal chamber ; 

 palps large, partly united behind ; foot large, compressed, keeled, without 

 groove or byssal sinus ; mantle margin smooth, duplex, more or less united 

 ventrally ; siphons well developed, usually with an epidermal tunic, internally 

 the branchial has an obsolete curtain valve, the sensory lamella running long- 

 itudinally on the inner surface of the mantle ; adductors peripheral, subequal ; 

 dioecious, marine. 



Shell porcellanous, with an obvious epidermis, usually rounded-triangular, 

 with smooth or concentrically sculptured surface, smooth margins and prom- 

 inent prosogyrous beaks ; valves equal, free, usually with a slight posterior 

 gape ; area not limited ; ligament variably external or internal ; resilium con- 

 necting subtriangular chondrophores usually excavated out of the hinge-plate, 

 rarely with a prop or buttress; hinge plate well-developed, with normally 

 an anterior and posterior lateral in the left, received into sockets or paired 

 laminae in the right valve, or obsolete ; cardinals in the right valve two, with 

 their dorsal edges usually soldered together, and one, bifid or deltoid in the 

 left, fitting below the former, a delicate accessory lamella often present in either 

 valve, or all may be more or less obsolete. 



Cretaceous to recent fauna. 



This group is so large and its extremes so variable that it is best divided 

 into subfamilies as follows : 



Afactritue, Typical. Ex. Cyinbophora, Mactra, Mactrella, Mactroloma, Spisula, Schizodes- 



ma, Mulinia, Rangia. 



Ptercpsidintz ; Pieropsis, Labiosa, Racta, Raetella. 

 Lutrariince ; Eastonia, Standella, Helerocardia, Tresus, Lulraria. 

 Xenatiintz ; Zenalia, Resania, Darina. 

 ? Anal in elliiuc ; Analinella. 



FAMILY ? CARDILIID^. 



Anatomy unknown. Shell cordiform, with prominent prosogyrous beaks, 

 small, thin, radially sculptured ; posterior adductor scar impressed upon a radi- 

 al myophoric lamina, the anterior scar elongated, the pallial line not sinuated ; 

 ligament external, seated on nymphs ; resilium internal, connecting projecting 

 chondrophores ; hinge without laterals, but the cardinal teeth as in Mactra. 



Tertiary to recent fauna. 



Ex. Cardilia. 



FAMILY MESODESMATID/E. 



Anatomical features resembling Mactra, but with the siphons naked and 

 not united. 



