MACTRIDZ. 157 
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, WM. Spengleri, Gray (cix, 
92-94). 
HEMIMACTRA, Swains., 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. MM. solidissima, 
Chemn. (cix, 95). M. triangula, Broce. (cix, 96). 
MACTROMERIS, Cony. Not characterized. MM. ovalis, 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 Mactra 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 Mactridz, 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-arelarge-and-very- 
robust, 
MULINEA, Gray, 1839.7 Shell oval-trigonal, subangular on each 
slope; cardinal teeth strong, laterals short and simple; internal 
ligament in the cardinal pit; pallial sinus angular, WM. edulis 
King (cix, 98-100). 53 
MACTRINULA, Gray, 18¥9. (Blainvillia, Hupé, 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. IM. plicataria, 
Linn. (cix, 1). $3 
MACTRELLA, Gray, 1849. Shell thin, triangular; cardinal teeth 
thin, lateral posterior tooth very short, rudimentary, and close 
to the cardinal tooth; marginal ligament triangular, separated 
from the cartilage-pit by a lamella; pallial sinus rounded, pro- 
found. WM. alata, Spengl. (cix, 2-4). 
HARVELLA, Gray, 184983 Shell thin, trigonal, well rounded 
inferiorly, posterior slope very narrow, carinated, its margin 
nearly straight; surface concentrically plicate; cardinal teeth 
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