INVERTEBRATE PALEONTOLOGY. 41 



species, must have rendered these mollusks very conspicuous objects among 

 the bivalves of the Cretaceous seas. 



Dr. Stoliczka states (Palaeont. Ind., Ill, 393) that the genus Inoceramus 

 "occurs from t lie Silurian to the close of the Cretaceous period." T think, 

 however, that this is a rather hasty conclusion; at least, it seems to me thai 

 it would be impossible to cite an example of any Silurian, or, indeed, 

 oilier Palaeozoic shell, that has yet been figured, certainly showing the hinge- 

 characters of Inoceramus. Woodward and d'Orbigny both give its range 

 from the Lias to the Chalk; and Pictet, also, states that it appears to be 

 exclusively characteristic of the Cretaceous and Jurassic. It seems to have 

 attained its greatest development during the Cretaceous period, and not to 

 have survived the close of* the same. 



The proper discrimination of species in this genus is extremely difficult, 

 not only on account of the great variations of form that different individuals 

 of the same species sometimes present, but also owing to the fact that we 

 rarely find perfect specimens with the opposite valves united ; while the right 

 and left valves of the same species generally differ more or less, and some- 

 times greatly, in form. In addition to this, the species are quite numerous, 

 and, where really distinct, often closely resemble each other. Their study is, 

 therefore, attended by nearly as many difficulties as would be met in attempt- 

 ing to classify the numerous species of the genus Unio, from merely separated, 

 water-worn, and mutilated valves. Because the task is a difficult one, how- 

 ever, is no reason why no attempt should be made to separate and define the 

 species, although we can scarcely hope to arrive at correct conclusions in all 

 cases. In the following review of the Upper Missouri Cretaceous species, 

 I have earnestly tried, to the best of my ability, to arrive at truth. That all 

 of the conclusions will lie sustained by those who come after me, and take 

 up the study of these shells with more extensive and better collections for 

 study, can scarcely be expected ; though it is believed that no material 

 changes will be found necessary. 

 6 n 



