578 GEOLOGY OF OHIO. 



lateral areas are ascribed to the diductor muscles. Ovarian markings have 

 not been detected. Vascular markings are rather common but are quite 

 variable. In all cases they consist of a few parallel low ridges extending 

 from the anterior border of the muscular area anteriorly about half the dis- 

 tance between the muscular' area and the anterior margin of the shell. 

 Here evidently the vascular tissues branched considerably. This is well 

 shown in some specimens, while in others the abnormal and at times very; 

 marked thickening along the margin of the shell, especially along 

 its anterior border, gives rise to a broad, often very thick border, against 

 which the aforesaid vascular markings abut, without giving further indica- 

 tion of their course. Usually two broad and very distinct elevations ex- 

 tend forward from the muscular area. Between these may lie a single, 

 much narrower ridge marked by a median groove anteriorly, or a broad 

 ridge of equal or even greater width may lie between them, and this eleva- 

 tion also is then marked by a median groove anteriorly; in some speci- 

 mens these median elevations are far more distinct and much broader 

 than the ridges between which they lie, increasing in width anteriorly, 

 whereas the lateral elevations, on the outside of this pair are but moder- 

 ately distinct. All of this seems to indicate the existence of a median 

 vascular tube, which at times did not approach the shell until near its 

 anterior extremity ; also the presence of a vascular tube on each side of 

 the median one, usually quite close to the shell. Oh the outside of these 

 lay perhaps on either side another vascular tube. From the last named 

 pair of tubes branches curved outwards and backwards towards the lateral 

 margins of the shell, branching dichotomously two or three times. From 

 the inner pair of vascular tubes a branch starts off laterally in the same 

 direction. From this branch of the inner vascular tubes, frop the tubes 

 themselves, and from the median tube, a series of branches start off an- 

 teriorly leaving on the shell, as casts, a series of reticulating grooves with 

 rather wide meshes, if this figure may be used. None of these vascu-, 

 lar markings continue to the very edge of the shell, and they are never 

 seen where the border is abnormally thickened. The degree of thicken- 

 ing of this border is sometimes remarkable and may reach a thickness of 

 4 mm., though this is shown only by a single specimen. In other shells 

 of the , c ame size the margin of this valve of the shell can hardly be said 

 to be thickened at all. It is difficult to assign a reason for such a great 

 variation in the thickening of the shell. It is usually accompanied by a 

 corresponding unusual thickening over the muscular area, but this is not 

 necessarily the case. 



This species is found at Soldiers' Home, Fauver's, Huffman's, and 

 Gentreville in the upper shaly courses. 



