OSTEACODA. 635 



Leperditia faljulites.] 



right valve in which a row of minute punctse is generally distinguishable; overlap 

 extending all around the free edges, strongest ventrally; except in rare instances, 

 neither valve has a flange or flattened border, and when present it is in all cases very 

 narrow and undefined; dorsal edge somewhat thickened, especially upon the left 

 side. Surface of valves smooth or very faintly pitted, rather evenly convex 

 with the greatest thickness somewhat beneath the center; a low ridge-like thicken- 

 ing along the posterior half of the dorsal margin of the left valve is to be noticed. 

 Eye tubercle just distinguishable in most cases, rarely so distinct as in the specimen 

 figured, often not to be detected. On the inner surface however it is always marked 

 by a distinct pit. Muscle spot not distinguishable externally except when the 

 specimens are weathered, but on the inner side it is often well marked and surrounded 

 by fine reticulating radial lines, short dorsally, longest post-ventrally. On the 

 inner side of the ventral edge of the right valve there are two rows of small papillae, 

 three to five in each, the number seeming to increase with age. The purpose of 

 these papillae, one series of which occurs in the anterior third, the other in the pos- 

 terior, evidently was to prevent undue overlapping of the valves by presenting an 

 obstacle to the entering ventral edge of the left valve. 



Of this species, I have before me no less than five hundred specimens, represent- 

 ing twelve localities in the states of Minnesota, Wisconsin, Illinois, Kentucky and 

 Tennessee. Considering its wide geographical range and abundance, it is remarka- 

 bly constant in all its characters. That it is so in its outer form is clearly enough 

 shown by the above measurements, taken from representative examples. They show 

 further that the northwestern specimens are on an average about one-third larger 

 than those from Tennessee. In all other respects however they are all practically 

 identical. 



In 1890 {loc. cit.) I believed it probable that L. josephiana Jones, would prove 

 distinct from L. fahulites, but it is now quite evident to me, as it also has become- to 

 Prof. Jones, that there is no ground whatever for any distinction between them. 

 Compared with other species, the Upper Silurian L. hisingeri Schmidt, is not far 

 removed, and the v&riQtj fabulina from Lake Winnipegosis very similar indeed. Still 

 as pointed out by Prof. Jones there are a number of minor differences between them, 

 and these will no doubt be added to when the later form is fully known. Another 

 closely related form is the L. wiluiensis Schmidt, from the Upper Silurian of Russia. 

 Its hinge line is shorter and the antero-ventral curve somewhat fuller, but in other 

 respects, even to the rows of punctae along the ventral margin, the two species are 

 much alike. L. linneyi Ulrich from the Upper Trenton of Kentucky is more obliquely 

 produced posteriorly and has a shorter hinge line, a flatter ventral edge, distinct 

 flanges, and better developed tubercle and muscle spot. 



