TRANSACTIONS OF WAGNER 

 1068 



TERTIARY FAUNA OF FLORIDA 



inflated and inequilateral valves ; anterior end short, subangular above, round- 

 ing evenly into the base below ; hinge-line forming a segment of a circle, and 

 except the anterior angle the outline of the valve is nearly suborbicular ; near 

 the umbo behind are traces of two radial ridges separated by a shallow sulcus, 

 but these rapidly become obsolete, and the surface of the valves smooth except 

 for incremental lines, which become stronger and more disposed in undula- 

 tions near the anterior base in senile specimens ; hinge normal, strong, the 

 lateral smooth and well developed, the left cardinal duplex, compressed, with 

 a small deep pit for the opposite cardinal below the junction ; anterior adductor 

 scar small, impressed, posterior scar much larger. Lon. 95, alt. 92, diam. 74 

 mm. 



This species is represented by two left valves in the National Collection, 

 obtained from North Carolina and Virginia. It forms a marked contrast to 

 /. fraterna in its nearly smooth subglobular form and greater size. It may 

 be that to specimens of this species Conrad referred when in his description 

 of /. rustica (= fraterna) he said that it " attains in North Carolina a larger 

 size than the /. cor with which Deshayes considers it identical." If Deshayes 

 had specimens of this sort his conclusion would not seem so unreasonable 

 as it does when one compares a good series of /. fraterna with /. cor (= hu- 

 mana). The present species, though very much less ponderous than /. fra- 

 terna, is thicker than /. humana and has its hinge less compressed, especially 

 the cardinals, of which the profile forms a broad M with a conical pit below 

 it ; the lateral is also stronger and proportionately more distant from the 

 cardinals; the posterior adductor scar is larger than in humana of the same 

 dimensions, while the umbo of /. Carolina is smaller, more pointed, less in- 

 volute, and is distant 6.5 millimetres from the hinge margin, while, in a 

 specimen of /. hnniana, slightly larger than that of /. Carolina, the umbo of 

 the same valve is eighteen millimetres from the margin. Correlatively, the 

 excavation in front of the beaks is considerably smaller in /. Carolina. The 

 largest senile specimens of /. humana are higher and less orbicular than the 

 types of /. Carolina, which are evidently senile specimens also. 



On the whole, in spite of the fact that the material is scanty, there seems 

 to be reason to think that in the Upper Miocene there is a type of Isocardia 

 leading from the older Miocene forms of Maryland in the direction of the 

 /. humana of the European fauna. The form figured by Hoernes from the 

 Vienna Miocene under the name of Isocardia cor (Moll., Wiener Beckens, 

 pi. 20, fig. 2 a-d, 1870) is in my opinion distinct from the recent shell, from 

 which it differs by its more produced and involute beaks, its much greater 



