388 GEOLOGY OF THE BLACK HILLS. 



a collection is examined. The species, as figured in the above-mentioned 

 work, appear distinct enough and the lines of demarkation well drawn 

 when a few individuals only are before one; but among the collections in 

 hand we find the boundries crossed in every direction, so that we are at a 

 loss in many cases for characters by which to separate the different forms 

 there indicated; while there are still others among them so entirely differ- 

 ent from any of those described and figured, that we are very reluctantly 

 compelled to consider them as entirely distinct. 



Among the forms representing those given under the names I. vanuxemi, 

 I. proximus, and I. proxunus var. subcircularis and also as I. convexus and 

 I. sagensis var. nebrascensis and I. balchii we find such gradually connecting 

 links as to make it extremely difficult, if not impossible, to draw lines of 

 distinction between them. Again, those resembling the forms given under 

 the names I. cripsii var. barabini and I. tenuilineatus, are similarly united. 



The forms which we had referred to I. sagensis and I. nebrascensis of 

 Owen vary so greatly in the posterior prolongation of the shell, and also 

 in the direction of the basal margin, as to defy all attempts to classify them 

 by their form; and in the relative degree of convexity and the comparative 

 distance of the surface undulations they are equally variable and unsatis- 

 factoiy. We find also that the relative projection of the beaks and the 

 width of the cartilage area, as well as the distance of the beaks from the 

 anterior end, is subject to great variation. Although in the collections 

 before us there are but few individuals which retain the right and left valves 

 in contact, still of many of them we have both valves of similar size and 

 form from the same locality and we find that the difference in convexity 

 between the two valves is not in any case very great, and often scarcely 

 perceptible. The beak of the left valve is usually larger, and projects more 

 beyond the line of the hinge than that of the right, and the cartilage area 

 is also a little wider. Perhaps if the real surface features of the shells were 

 preserved on the specimens, more reliable characters for specific distinction 

 might be presented, but the fibrous coating is almost always absent, being 

 generally left in the matrix, and not collected; or perhaps in many cases 

 removed by some action of decay before the shells are filially imbedded in 

 the rocky sediment, so that in collections it is seldom that any part of this 



