FOSSILS OF THE HUDSON" RIVER GROUP. 97 



the cardinal margin to the crest of tlie umbonal ridge, where they become 

 obsolete. 



This species is readily distinguished from any of the other forms asso- 

 ciated with it, by the strong, distant, and oblique plications of the car- 

 dinal slope, which is a marked feature. 



The original figures of this species, given in Volume I., Palaeontology 

 of New York, as above cited, have been copied by several European 

 authors, and referred to as an example of the genus Orthonota, Conrad- 

 It is scarcely necessary, however, to say that it differs very materially 

 from the true type of that genus, Orthonota imdulata, Conr., which has a 

 hinge line, -perfectly straight, extending beneath the beaks and reaching 

 to the extremity of the anterior end of the shell. The undulations of 

 that shell, as well as the secondary ridges of the cardinal slope, differ 

 very materially from the short, oblique plicse marking this species. The 

 genus Orthonota consists of the first or earliest solen-like shells of which 

 we have any knowledge, and the earliest. form in which there is the 

 slightest chance of finding a pallial sinus — the pallial line in all the 

 forms which we know in rocks of earlier date being without sinus — 

 and we are not certain in this one or even in any Devonian genus, 

 of a truly sinuate pallial line. 



CEPHALOPODA. 



Genus ORTIIOCERAS, Breyn. 

 Orthoceras Duseri (n. sp.). 



Plate 3, figs. 2-4. 



Shell of medium size, rather rapidly and gradually enlarging from 

 below upwards, the diameter increasing to twice the size in the space of 

 four and a half inches. Transverse section circular ; length of the outer 

 chamber not determined. Septa moderately concave, and closely ar- 

 ranged, but gradually increasing in distance with the increased size of 

 the shell — six chambers occupying the space of one inch where the 

 diameter of the shell is one and a half inches at the uj^per one of those 

 measured; nearer the joint there are ten to twelve in the same distance 

 where the diameter is only three-fourths of an inch. Siphuncle eccen- 

 tric situated a littie nearer to the center than to the margin; very small 

 where it passes through the sej^ta, but expanding within the chambers 



7 



