TRANSACTIONS OF WAGNER 

 I070 



TERTIARY FAUNA OF FLORIDA 



or spiny, and glabrous groups, the latter being equivalent to Isocardia. The 

 following year Lamarck, in his Prodromus, selected C. aculeatum Linne as 

 an exemplar of the genus. 



From that time the work of systematists has been confined chiefly to 

 naming minor groups characterized by peculiarities of sculpture, many of 

 which are of little structural importance and chiefly convenient as a means 

 of assorting a rather uncomfortably numerous assembly of species. 



The genus Cardium in the wider sense is very homogeneous compared with 

 most large groups. The differences are of comparatively minor importance if 

 judged by the standard of many other analogous groups. 



The sculpture is predominantly radial, only in very exceptional cases does 

 concentric or oblique sculpture or marked reticulation appear. 



The hinge throughout the typical portion of the group is very uniform, 

 comprising lateral lamellae in both valves and two cardinals in each valve, of 

 which, when interlocked, the inner pair are more robust and the outer pair 

 feeble, so as to be liable to be overlooked or to become obsolete in worn or 

 senile specimens. A more important character is that brought about by 

 " torsion," or a process of twisting, which results in many species in bringing 

 the two cardinals of one valve one above another, vertically, while in the 

 opposite valve the cardinals will follow each other in a horizontal line. The 

 teeth often, especially in thin species, seem to spring from the umbonal cavity 

 rather than from a hinge-plate, a feature which I have tried to indicate by 

 the term Cyclodont. Another feature, perhaps connected with the apparent 

 rotation of the cardinals, is the tendency of the dorsal margin, just in front of 

 the umbones, to be pouted and thickened. 



In all the typical Cardiums there is neither lunule nor escutcheon, though 

 a space between the terminal ribs and the shell margin may be smoother and 

 simulate a lunular area. The lateral teeth are present in all groups except the 

 subgenus Lophocardiuiii. In Serripes alone the cardinals are obsolete. All 

 the species, especially those of tropical waters, tend to have the ends of the 

 shell, especially the posterior end, slightly different in sculpture from the 

 middle of the disk. Thus in Lccvicardiuni the ends are smooth and the disk 

 obsoletely radially grooved, in Serripes the exact reverse is the case. Whether 

 the surface be smooth or not, there is always some serration of the internal 

 basal margins. In Protocardia a strongly differentiated posterior area is 

 developed, the sculpture of which is doubtless correlated with the structure 

 of the mantle edge around the siphonal apertures. In Tropidocardiuni there 

 are several channels (corresponding, not to elevations of sculpture, but to 



