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865 



TERTIARY FAUNA OF FLORIDA 



become obsolete in the adult, or are represented only by frail remnants. One 

 such is between the side of the cardinal tooth and the surface of the resilium 

 in the left valve (4$ of Bernard's notation); others are parallel to the arms of 

 the cardinal teeth in the right valve. These thin sheets of shelly matter in 

 the adult I call " accessory lamellae." As might be expected, they are very 

 inconstant and excessively fragile if well developed ; usually, whether by 

 breakage or otherwise, they appear merely as low ridges parallel to the normal 

 teeth and laminae. These lamellae are often referred to as " teeth" in descrip- 

 tions of species, and, to the rather frequent presence of the accessory lamella 

 (4 b) of the posterior arm of the left cardinal tooth is probably due the ascrip- 

 tion by the older authors of three cardinal teeth to the valve in the genus 

 Mactra. 



When the hinge-plate is excessively oblique, as in Mactra alata Spengler, 

 the thin and slender teeth are sometimes reinforced by horizontal or vertical 

 buttresses, which extend from the teeth to the hinge-margin. In using ad- 

 jectives denoting the direction of plane surfaces in this paper the shell is con- 

 ceived of as suspended by the umbones with its longest antero-posterior line 

 in a horizontal plane. When, therefore, a buttress extends in a plane sub- 

 stantially parallel with the plane including the margin of the valve, it may 

 cover part of the sinus so that the portion covered is either wholly filled with 

 shelly matter or is merely roofed over by a shelly plate. The former condition 

 is more common. In other cases the buttresses may extend in a plane at 

 right angles to 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 cavities, those nearer the beaks being cellular. This variety of buttress 

 I call a "septum." (See pi. 27, fig. 14^.) It is somewhat rare, and when 

 present curiously complicates the hinge. The initiation of hinge-teeth is 

 illustrated in a curious way in Schi.witcsina Speiigleri, where the ridge sup- 

 porting 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 very little encouragement to develop into an entirely 

 new type of tooth, at least compared with the primitive teeth of the hinge of 

 Mactra. 



In the situation and form of the different teeth upon the hinge-plate the 

 influence of the different strains and stresses involved in the mechanical action 

 of the hinge are clearly discernible if intelligently looked for. A study of 



