TRANSACTIONS OF WAGNER 

 868 



TERTIARY FAUNA OF FLORIDA 



The lateral laminae may be set frankly on the hinge-plate, or more or less 

 confluent with its ventral margin ; usually long, slender, and simple, they are 

 sometimes fluted longitudinally with the distal angle recurred. The pafred 

 laminae are more variable than the single ones. The former vary from little 

 elevated ridges on either side of an indentation in the substance of the hinge- 

 plate, to thin, sharply defined lamelke ; the dorsal one of the pair may be 

 nearly confluent with the dorsal margin of the valve, or it may be a clean-cut, 

 independent lamina rising sharply from the hinge-plate. In the more trian- 

 gular species the laterals are shorter and closer to the beaks, and in some of 

 the very thin-shelled forms (such as Pleropsis or Labiosd), where no great 

 strain is ever brought upon them, they are always imperfectly developed. In 

 Harvella, however, we find them clearly defined, notwithstanding the thinness 

 of the shell. In some groups (such as that typified by Spisula solidissiiiur) 

 the tendency of the valves to rotation on the resilium as an axis is opposed 

 by the development of transverse grooves on the opposed surfaces of the 

 laterals, and in a few species this grooving has become so pronounced that 

 the valves can hardly be separated without the use of force sufficient to 

 fracture the laterals. It is an illustration of the same principle which de- 

 veloped the hinge of Area, but applied secondarily upon a type of hinge 

 which, when adult, is the exact antithesis to that of the Prionodont. 



In studying the development and mutations of the cardinal teeth, besides 

 the changes which result from the dorsal coalescence of previously distinct 

 parts, another set of variations present themselves which a complete series 

 of the stages of growth in any single type would doubtless show to be dyna- 

 mic. It is obvious, in species with sagittate ligaments, that the sinus between 

 the barbs of the ligament is filled by a pointed process of each valve, forming 

 part of the dorsal margin, which extends backward, and in a single valve is 

 seen to be situated over the resiliary pit, or partially so. This may be called 

 the cardinal spur. Its tip is sometimes slightly recurved or callous. The 

 cardinal teeth are situated under the anterior part of the spur, and in mature 

 specimens the posterior arm of the cardinal tooth in the right valve is often 

 coalescent with the spur. In that group which has the ligament partly or 

 wholly walled off from the cartilage-pit by a shelly ridge, this ridge unites 

 the spur with the dorsal shell margin, between the scar of the ligament and 

 the pit, or chondrophore, properly so called. 



Now the dynamical feature to which I would direct attention in con- 

 nection with the cardinal teeth, especially in the right valve, is that the two 



