" BALL : MACTRID^ AND MESODESMATIDiE. 207 



the plane of the valves, and inclined at any angle to the plane of 

 their transverse diameter which will give the greatest strength with 

 the least expenditure of material. Such buttresses cut the sinus into 

 two or more ca\-ities, those nearest the beak being cellular. This 

 variety of buttress I call a "septum." It occurs in Mactrella, but 

 is somewhat rare, and, when present, curiously complicates the hinge. 



The initiation of hinge-teeth is illustrated in a curious way in 

 Schizodesma Spengleri, where the ridge supporting the ligament is 

 produced at the margin of the valve into an obscure prominence which 

 is partly received by a slight depression in the opposite valve. This 

 requires only a little encouragement to develop into an entirely new 

 type of tooth, at least compared with the normally present teeth of 

 the hinge of Mactra. 



With the separation of the primitive ligament into ligament and 

 resilium, a more or less marked space often intervened between 

 their adjacent sides. In the typical Mactras, this space has become 

 more or less occupied by a shelly ridge, which, when the valves are 

 closed, partially cuts off the ligament fi'om the resilium. This ridge, 

 or shelly wall, naturally belongs to the posterior slope of the shell, 

 and may become coalescent, over the chondrophore, with the spur. 

 In a small antipodean group of species the partition is accomplished 

 in another way. The spur projects, and is continued as a more or 

 less irregular shelly rod, which is laid close to the ventral border of 

 the ligament, and is attached to the shell, though not thoroughly 

 coalescent. Mactra ovata, Gray, oilers a good example of this for- 

 mation. In a single species (which appears to be M. tristis, Gray, of 

 Reeve's Conch. Iconica) the ligament, while quite separate from the 

 chondrophore, has itself sunk below the dorsal shell-margin, only its 

 most anterior point remaining at the surface. 



The shelly portions of the hinge arise from a shelly basis stretched 

 antero- posteriorly between the limbs of the arch forming the cardinal 

 margin. This basis is called the hinge-plate, and it may have its 

 surface "flat," i.e. nearly parallel to a vertical plane between the 

 valve margins, or "oblique," that is inclined at an angle, so that its 

 dorsal edge starts from the valve some distance from the dorsal 

 margin of the latter. When the hinge-plate forms a marked angle 

 with the valve, the space between the ventral edge of the plate and 

 the dorsal margin of the valve is said to be "excavated," forming a 

 A- shaped valley on each side of the chondrophore ; a state of things 

 some of the older writers have tried to indicate by the objectionable 

 expression that the hinge is " double-edged." In Mactra the hinge- 

 plate is never perfectly flat (as we find it, for instance, in Astarte) ; 

 and in some of the thin-shelled forms, like Pterop&is or Lahiosa, the 

 excavation is deep and sharp, and is indicated on internal casts of the 

 fossils by two areas set off from the general mass by deeply incised 

 lines. A hinge-plate is always present, however, and we never find 

 a Mactroid hinge set directly on the margin of the valve as in Area. 

 In some very thin and small forms the edge of the hinge -plate itself is 

 turned up, and becomes patulous to form the laterals {JiaeteUa). 



The diverging, more or less coalescent, primitive teeth which form 



