420 MOLLUSCA SUB-KINGDOM VI 
Superfamily 15. MACTRACEA. Gray. 
Resilium internal, seated on chondrophores, left cardinal tooth bifid, fitting below the 
two right cardinals, which are more or less joined together dorsally. Inner wall of the 
mantle behind the siphons exchiliting a laminar sense organ. 
Family 38. Mactridae. Gray. 
Shell porcellanous, with an obvious epidermis, usually rounded-triangular, with 
smooth or concentrically sculptured surface, smooth margins, and prominent prosoqgyrous 
beaks ; valves equal, free, usually with a slight posterior gape; area not limited ; liga- 
ment variably external or internal; resilium connecting sub-triangular 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 cardinal in the 
left, fitting below the former, a delicate accessory lamella often present in either valve, or 
all may be more or less obsolete; siphons well developed, united, and usually with an 
epidermal tunte ; adductors peripheral, sub-equal. Cretaceous to Recent. 
This group is so large and its extremes so variable, that it is best divided into sub-families, 
as follows :'—Mactrinae, Pteropsidinae, Lutrariinae, Zenatiinae, and ? Anatinellinae. 
Mactra, Linn. (Fig. 775). Ligament and resilium 
separated by a shelly septum. Tertiary and Recent. 
Sub- genera: Jactra s.s., Coelomactra, Mactroderma, 
Mactrotoma, Dall ; Mactrella, Gray. 
Spisula, Gray. Ligament and resilium not separated, 
the former more or less external. Cretaceous to Recent. 


Fic. 775. 

_Mactra Podolica, Kichw. Miocene 
(Sarmatian Stage); Wiesen, near 
Vienna. Lutraria elliptica, Roissy. Pliocene; Rhodes. 2/s. 
Fic. 776. 
Sub-genera: Hemimactra, Swains. ; Leptospisula, Dall; Cymbophora, Gabb; Schizodesma, 
Gray. 
Mulinia, Gray. Ligament and resilium immersed in the same socket. Miocene 
and Recent. 
Rangia, Desm. (Gnathodon, Gray, non Goldfuss). Like Mulinia, but with 
elongated laterals, and the anterior lateral hooked at the umbonal end. Estuarine. 
Pteropsis, Conrad. Eocene. Labiosa (Schmidt), Méler. Miocene and Recent. 
Lutraria, Lam. (Fig. 776). Soleniform, hinge Mactroid. . Tertiary and Recent. 
Tresus, Gray (Schizothaerus, Conr.); Eastonia, Gray ; Heterocardia, Desh. Tertiary 
and Recent. Zenatia, Gray; Anatinella, Sow. Recent. 
1 Cf. Dall, W. H., Synopsis of a Review of the Genera of Recent and Tertiary Mactridae and 
Mesodesmatidae (Proc, Mal. Soc. vol. I. pp. 203-213), March 1895. 
