INVERTEBRATE PALAEONTOLOGY. 53 



the axis of the umbones; beaks moderately convex, rising little above the 

 hinge, oblique, scarcely incurved, located about one fifth the horizontal 

 diameter (length) of the shell behind the anterior border. Surface orna- 

 mented by regular, distinct, concentric undulations. 



Length, about 5.70 inches; height, 5.60 inches. These measurements 

 are taken from a medium-sized specimen. 



A careful study of our specimens, in connection with Dr. Owen's figures 

 and descriptions of his I. Sagensis and /. Nebrascensis, has led to the 

 conclusion that the forms for which these names were proposed are nol 

 specifically distinct; the type of the latter being only a larger individual of 

 very slightly different outline from that for which the name / Sagensis was 

 proposed. 



It will be seen, however, that the form here referred to I. Nebrascensis 

 is a little more rounded, or less transverse, than the curves of the undulations 

 on Dr. Owen's figure would indicate for the outline of his type. There 

 appear, however, to be so many gradations in characters of this kind that to 

 separate the form we have figured, as a variety or species, would require that 

 we should, in the same way, separate almost every individual specimen .under 

 a different name. 



In comparing our figure with those given by Dr. Owen, however, it 

 should be remembered that his figures are turned obliquely with the beak 

 upward. In order to make the comparison understandingly, both figures 

 should be placed with the hinge-line upward and horizontally arranged. In 

 making the comparison in this way, it will be seen that there is less differ- 

 ence between the form of the shell under consideration, and Dr. Owen's type 

 of I. Nebrascensis, than would appear to exist on a hasty examination. 



Locality and position. — White River, Nebraska, above Bad Lands; from 

 the Fort Pierre group of the Cretaceous. 



Inoeera m u s proxi in n s , Tuomey ?. 



Plato 12, figs. T, a, b. 



Inoceramus proximus, Tuomey (1854), Proceed. Aoad. Nat. Sci. Pliilad., VII, 171. 

 Inoceramus Mortoni, Meek and Hayden (Oct., 1860). in catalogue, Proceed. Acad. Philad., XII, 428. 

 Compare J. coitfertim-annulatus, V. Roemer (1852), Kreid. von Texas, pi. vii, fig. 4. 



Shell (righl valve) subcircular when young, but becoming transversely 

 suboval with age, much compressed: posterior side considerably longer than 



