Description of Five New Species of Silurian Fossils. 315 



in the cast on each side of the cardinal line, shows the thick and 

 firm character of the shell at the hinge line. 



A large specimen has a length of 2 2-10 inches ; thickness, 1 2-10 

 inches, and height abour the same. The specimens differ a great deal 

 in size. 



There are indications on the casts of concentric lines, with which the 

 surface of the shell was probably ornamenled. 



Collected by W. C. Egan, of Chicago, Illinois, in rocks of the age of 

 the Niagara Group, at Bridgeport and Cicero, Illinois. 



Ambonychia robusta, n. sp. 



Plate YIII. , fig. 3, view of the riglit side of a cast, tliough too much depressed in appear- 

 ance, and not showing as it should the abrupt bending over of the shell on the anterior side; 

 fig . 3a, view of the anterior side of the shell . 



Shell large, very ventricose, anterior side flattened and depressed in 

 the region of the byssus; beaks acute, triangular; posterior wing, 

 short, muscular impression high, and surface marked by about forty 

 simple radii, which are crossed by fine concentric striae. 



This species is distinguished by its large size, triangular beaks, 

 flattened and depressed anterior side, and high position of the muscu- 

 lar impression. 



The specimens are quite variable in size, but a medium specimen has a 

 height from base to beak of 2 6-10 inches and thickness of 1 8-10 inches. 



Specimens having the shell quite well preserved, occur in the upper 

 part of the Hudson River Group, near Osgood, Indiana, where I col- 

 lected the type specimens. And I refer the large casts having a flat 

 anterior side, which occur at Richmond, Indiana, to the same species, 

 though they have a greater height in proportion to their length and 

 thickness. 



(?)— — (?)• 



Plate VIII., fig. 4, view of two joints or parts of a peculiar fossil; fig. ia, a sectional view 

 showing the radiating or fibrous structure. 



This singular fossil was- collected by W. J. Paterson, Esq., in the 

 Hudson River Group, at Cincinnati. He found six joints or sections. 



The structure is not that of the shell of a Cephalopod, nor indeed that 

 of an}^ other fossil shell with which I am acquainted. At first glance one 

 is likely to suggest that it is the siph uncle of an Orthoceras, but a little 

 inspection dispels this illusion. The thickness is nearly a quarter of 

 an inch, and appears to be composed of fibres, not unlike in appear- 

 ance some specimens of stalagmite. I do not, however, see how it can 



