30 



in arriving at any very satisfactory conclusions as to the limits of species 

 ■when so large a collection is examined. The species, as figured in the 

 above-mentioned work, appear distinct enough, and the lines of demarka- 

 tion well drawn, when a few individuals only are before one, but among 

 the collections in hand we find the boundaries crossed in every direc- 

 tion, 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 different from any of those described and fig- 

 ured that ^e are very reluctantly compelled to consider them as entirely 

 distinct. 



Among the forms representing those given under the names I. 

 Vaunxemi, I. proximus, and I. proximus, var. suboireularis, and also 

 as I. convexus and I. Sa^ensis, var. Nebrascensis and /, 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 re- 

 sembling the forms given under the names I. Cripsii, var. Barahini and 

 J. tennilineatus, are similarly united. 



The forms which we had referred to 1. Sagensis and I. Nebrascensis 

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

 also ih the direction of the basal margin as to defy all attempts to class- 

 ify them by their form, and in the relative degree of convexity and the 

 comparative distance of the surface undulations they are eqally variable 

 and unsatisfactory. 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 collection before us there are but few individuals which retain 

 the right and left valves in contact, still of many of them we have both 

 vailves 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 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 finally imbedded in the rocky 

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

 is retained on the specimens, except a little around the beaks or along 

 the hinge margins. Owing to these difficulties, the same that Mr. Meek 

 has found to exist among his specimens, and also to the impossibility of 

 giving a sufficient number of figures to show fully and clearly the vari- 

 ations which we have found among the examples in hand, we shall be 

 obliged to follow very nearly the same specific limits which that author 

 has found desirable, though we must confess that in some cases we 

 should prefer rather to consider some of the forms found associated to- 



