FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 65 



concave mesial sinus more or less developed, and the dorsal valve a mesial fold 

 corresponding thereto. Some of the species, as S. laevis and S. microcamera 

 have the hinge-line straight and much extended. 



Stricklandinia louisvillensis. N. SP. 



Plate XXXIV., figures 81, 32, 33 and 34. 



Shell of medium size, sub-circular or sub-elliptical ; width greater than 

 length ; maximum width, below middle of shell, nearer to base than shown in 

 the figures 31 and 32; cardinal line straight on each side of beaks, but deflecting 

 somewhat to front ; shorter than the width of the shell, and its extremities 

 rounded ; lateral margins almost straight, or only slightly curved ; at their 

 basal termination the shell attains its greatest width ; basal margin broadly 

 curved, with the exception of its central half, which is somewhat produced, and 

 forming a broad but short linguif orm basal extension, giving the shell a some- 

 what trilobed aspect ; both valves are about equally convex, ventral umbo a 

 little more than dorsal ; umbones and beaks are so slightly developed as to give 

 only a very moderate angulation to cardinal line at its center. The hinge- 

 areas are almost concealed by the close approximation of the beaks; that of 

 ventral valve shows a narrow, almost linear strip along its rounded margins. 

 Neither of the valves shows the least indication of either mesial fold or sinus, 

 and the only point of distinction between the two valves is the umbo of the 

 ventral valve, which is slightly more convex than the dorsal, and which is also 

 a little elevated above the opposite beak. The surface is ornamented by seven 

 low, rounded plications on each valve, which start below the umbones and 

 extend, gradually increasing in strength and distance, to basal margin ; trans- 

 versely' they are limited to middle lobe of shell; lateral lobes and umbones are 

 entirely smooth or free from ribs ; this feature, and the three-lobate character 

 in the basal margin, are not sufficiently expressed in the figures, which also 

 show one rib too much. Of the seven plications on each valve, the central 

 three are considerably stronger than the lateral ones. The whole surface of 

 the shell is covered by fine, closely set, concentric striae of growth, of which 

 a few are slightly stronger than the balance, dividing the surface into several 

 concentric zones ; there are also radiating lines visible which are still finer than 

 the concentric striae. This shell bears some resemblance to Stricklandinia 

 davidsoni of Billings, as figured and described by him in his "Palaeozoic 

 Fossils, Vol. II.," but it differs from that species by having its greatest dimen- 

 sion transversely, while the other shell measures most longitudinally ; in our 

 specimen the middle lobe, or the basal extension, is broader in proportion to the 

 whole width of the front, but considerably shorter than that of Str. davidsoni ; 

 and the most prominent points of distinction are the plications, which in S. 



GEOL. SUB. 9 



