TRANSACTIONS OF WAGNER 

 S64 



TERTIARY FAUNA OF FLORIDA 



i.e., nearly parallel to a vertical plane between the closed valve-margins, or 

 " oblique" that is, inclined at an angle so that its dorsal edge starts from the 

 valve within 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 

 V-shaped valley on each side of the chondrophore, a state of things which 

 some of the older writers tried to indicate by the objectionable expression that 

 the hinge was " double-edged^ In Mactra the hinge-plate is never perfectly 

 flat (such as we find, for instance, in Astarte), and in some of the thin-shelled 

 forms, like Ptcropsis or Labiosa, the excavation is deep and sharp, and is in- 

 dicated on internal casts of the fossils by two areas set off from the general 

 mass by deeply incised lines parallel with the dorsal margin. A hinge-plate 

 is always present, however, and we never find a Mactroid hinge set directly 

 upon the cardinal margin, as in Area. 



As the shelly projections usually called lateral teeth are very variable 

 and often ill-defined compared with the cardinal teeth, I have found it con- 

 ducive to clearness to term them " lamina;" rather than " teeth." The cardinal 

 teeth are essentially in origin like the teeth of Cyrciia, but in the process of 

 evolution the two outside teeth have gained strength by retaining a union at 

 the angle where they join, and the inner bifid tooth (that of the left valve) has 

 become more triangular, in harmony with the bearing surfaces of its neigh- 

 bors. That this is not merely a speculation may be seen by comparing a 

 regular Mactra with Rangia or some of the Mulinias which do not continue 

 to unite the outside cardinals, and it will be seen that the form of the inner 

 cardinal is much more triangular and constant in its shape in the first men- 

 tioned. The study of very young shells has shown in every case that the 

 arms of the cardinal tooth of the right valve in Mactra, after separating from 

 the inner ends of the laminse, are distinct teeth, and in Mulinia and some 

 species of Mactra, as well as Rangia, they remain separated, more or less 

 completely, even in the adult. The diverging branches of this compound 

 tooth I call its anterior and posterior "arms." The single tooth in the oppo- 

 site valve is sometimes excavated in the middle line and curved upward like 

 the petal of a lily ; such teeth I name " petaloid." 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; the other one, behind the cartilage- 

 pit, the "posterior sinus." The space between the arms of the cardinal tooth 

 is the " ventral sinus." There are some parts of the nepionic laminse which 



