FOSSILS OP THE SILURIAN AND DEVONIAN ROCKS. 197 



Surface marked by fine and coarser, irregular, lamellose lines of growth, and 

 fine, regular, undulating, elevated, thread-like striae, parallel to the lines of 

 growth ; longitudinally marked by rounded, or sometimes sharper, undulating, 

 revolving striae, of which there are five or six in the space of five millim- 

 eters. 



These markings are visible on the macerated inner laminae of the shell, and 

 often even on the internal cast. The internal cast is strongly marked by the 

 suture-lines, which are deeply impressed from the solution and removal of the 

 imbricating margins of the septa. 



The largest known specimen of this species has a diameter of 360 millimeters, 

 and the chamber of habitation of the same specimen, measured along the mid- 

 dle of the lateral face, has a length of more than 360 millimeters, and measured 

 on the periphery, has a length of more than 450 millimeters. This species 

 resembles N. oriens and magister, from both of which it is, however, easily 

 distinguished. 



Formation and Locality. Found in the hydraulic limestone of the Devonian formation at the 

 Falls of the Ohio, in Kentucky and Indiana. The figure on plate 24 is only one-half of the natural size 

 of the shell from which it was taken. 



MOLLTJSCA. 



LAMELLIBRANCHIATA. 

 Genus Limoptera. Han. 



Limoptera, Hall. Prelim. Notice Lam. Shells 1870. 

 Etymology : lima, a genus of shells; pteron, a wing. 



Shell large, inequilateral, inequivalve ; the right valve the smaller ; ligamen- 

 tal area large, common to both valves ; longitudinally striate, and extending 

 to the extremities of the wings. Margins of valves sinuate on the anterior 

 border, forming an elongate byssal opening ; hinge edentulous (?). Anterior 

 muscular impression situated within the umbones, very small and deep ; the 

 posterior one large, and placed a little behind the middle of shell, and nearer 

 to the hinge than to base of valve. Pallial line simple, formed of a series of 

 small, deep pits (or, as seen in the casts, composed of a' series of small 

 nodes). 



The general form of the shells, when well preserved, is broadly ovate or 



