MACTRIC^. 15t 



marginal ligament triangular, separated in the pit by a testaceous 

 lamina ; pallial sinus rounded. 



SCHIZODESMA, Gray, 1837. Ligament not distinctly separate 

 from the cartilage (the character does not appear to be very 

 constant); pallial sinus angular. M. Spengleri, Gray' (cix, 

 92-94). 



HEMiMACTRA, Swaius., 1840. (Spisula and Standella, Gray, 

 1849.) Shell trigonal; posterior slope more or less carinated; 

 cardinal teeth moderate, laterals elongated ; ligament triangular, 

 submarginal, not separated by a testaceous lamella from the 

 cardinal pit ; pallial sinus small, rounded. M. soUdissima^ 

 Chemn. (cix, 95). M. triaiigula, Brocc. (cix, 96). 



MACTROMERis, Conr. Not characterized. M. ovaliSj Gould 

 (ex, 22). 



oxypERAS, Morch, 1853. Proposed for the more elongated 

 triangular forms of Hemimactra, concentrically plicate-striate. 

 M. triangularis^ Lam. (cix, 97). 



CYMBOPHORA, Gabb, 1869. Proposed for Macb^a Ashburnerii, 

 a cretaceous species. Form as in the typical Mactra, but the 

 hinge is composed of a rather heavy hinge-plate, bearing a 

 cartilage-pit, not sunk into its substance, as in other genera of 

 the Mactridae, but, as it were, built upon its surface ; a small 

 delicate spoon-shaped process laid obliquely under the beaks, 

 its base being on or slightly above the level of the hinge-plate ; 

 in the right valve the cardinal tooth is single, very delicate, and 

 nearly at a right-angle with the anterior wall of the cartilage- 

 pit ; in the left valve the tooth is A-shaped, entirely separated 

 from the pit, very slender, and articulates between the tooth and 

 the pit of the opposite side ; the lateral teeth are large and very 

 robust. 



MULINEA, Gray, 1836. Shell oval-trigonal, subangular on each 

 slope ; cardinal teeth strong, laterals short and simple ; internal 

 ligament in the cardinal pit ; pallial sinus angular. M. edulis^ 

 King (cix, 98-100). 



MACTRiNULA, Gray, 1849. (Blainvillia, Hupe, 1854.) Shell 

 thin, trigonal, posterior slope shortest, angulated, cardinal teeth 

 thin, the short laterals close to them ; outer ligament separated 

 from the cardinal pit by a testaceous lamella. M. plicataria, 

 Linn, (cix, 1). 



MACTRELLA, Gray, 1849. Shell thin, triangular ; cardinal teeth 

 thin, lateral posterior tooth yery short, rudimentary, and close 

 to the cardinal tooth ; marginal ligament triangular, separated 

 from the cartilage-pit by a lamella; pallial sinus rounded, pro- 

 found. 31. alata, Spengl. (cix, 2-4). 



HARVELLA, Gray, 1849. Shell thin, trigonal, well rounded 

 inferiorly, posterior slope very narrow, carinated, its margin 

 nearly straight ; surface concentrically plicate ; cardinal teeth 



