206 I'KOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



right valve wliifh receive the hiteral teeth of the left valve between 

 them "lamiuaj," and only the laterals of the left valve "laterals"; 

 by keeping to which plan a good deal of verbiage is avoided, and the 

 meaning is made clearer. The angular space between the anterior 

 arm of the cardinal tooth and the dorsal margin of the valve I call the 

 "anterior sinus" of the hinge, and the other, behind the chondrophore, 

 the "posterior sinus." The space between the arms of the cardinal 

 tooth is the "ventral sinus." 



When shell-matter is being secreted from the pallial surface for the 

 growth of the hinge, there are some unoccupied crevices to which it 

 may penetrate by capillaiy action. One such is between the side of 

 the cardinal tooth and the surface of the resilium in the left valve ; 

 others are parallel to the arms of the cardinal tooth in the right 

 valve. In these places thin sheets of shelly matter may be deposited 

 ■which I call " accessory lamellae," which are often confounded in 

 descriptions with true teeth. They are, as might be expected, 

 excessively fragile and very inconstant; sometimes they exist un- 

 attached, like a species of pearl. To the presence of these lamellae 

 is due the ascription, in old works, of three cardinal teeth to the valve 

 in Mactra. Keally there is one true cardinal in the left and two in 

 the right valve, normally, in this group ; to which may be added an 

 anterior and posterior accessory lamella in the left, and an anterior 

 accessory lamella in the right valve ; all of which lamellae may be 

 present, and any one of which may be absent in any species, or in 

 some individuals of any species ; though the anterior lamellae are 

 tolerably constant. These lamellae may be attached to the hinge- 

 plate by a slender peduncle, may be solidly rooted like a true tooth, 

 or may be mounted upon the edge or side of a lateral tooth and 

 coalescent with it. In the latter case a close and intelligent inspec- 

 tion will almost always result in the detection of the elements 

 belonging to the tooth and to the lamella respectively. In Madrotoma 

 we have the most marked instance of this, since in the right valve the 

 anterior lamella is mounted on the back of the ventral lamina, and 

 the anterior arm of the cardinal is (in its turn) mounted on the back 

 of the accessory lamella. A profile \'iew shows the upturned point of 

 each still remaining separate ; while a vertical view reveals the 

 sutures at which the separate masses coalesced. In the left valve 

 only the lamella and the anterior lateral are coalescent. 



^\Tien the hinge-plate is very obliquely set on to the shell, as in 

 Mactra alata, Sj)engler, the thin and slender cardinal teeth are some- 

 times reinforced by horizontal or vertical "buttresses," which extend 

 from the teeth to the liiuge-margin. In using adjectives denoting the 

 direction of plane surfaces in description, I conceive of the shell as 

 suspended by the beaks of the valves, with its longer antero-posterior 

 line in a horizontal plane. When, therefore, the buttress extends 

 in a plane substantially parallel with the plane which includes the 

 margin of the valves it may cover part of the sinus, so that the portion 

 covered is either wholly filled with shelly matter, or is merely "roofed 

 over" with a shelly plate. The former condition is more common. 

 In other cases the buttresses may extend in a plane at right angles to 



