PALZ ONTOLOGY. 117 
Locality and formation.—Occurs abundantly with A. longispinus in the crinoidal limestone 
near junction of the two Colorados, and in the same rock, sixty miles west of that point, near 
the Great Cafion of the Colorado. 
ARCHOCIDARIS GRACILIS, (n. sp.) 
Plate I, figs. 4 and 4a. 
Form of interambulacral plates unknown. A fragment bearing the central boss shows that 
the circle surrounding the articulating tubercle is ornamented with a single row of small 
tubercles, and that the articulating tubercle is deeply perforated. 
Spines straight or curved, slender, elongate, conical, scarcely fusiform, with a circular section 
throughout; diameter of all parts much less than that of the ring. Surface thickly set with fine 
granulations, toward the base separated by spaces scarcely greater than their diameters; nearer 
the summit they are more scattered. Ring prominent, crenulate (?) in my specimens encircling 
the spine at right angles to its axis. 
This delicate and beautiful species is scarcely likely to be confounded with any other known. 
Its slender, conical form and fine and numerous granulations separate it widely from its asso- 
ciates, as well as from other described species of which I have any knowledge. 
Locality and formation.—Occurs with the last. 
AMMONITES. Brug. 
AMMONITES PERCARINATUS, Hall & Meek. Memoirs Amer. Acad. Arts & Sciences, 1856, p. 396. 
Pl. IV, figs. 2 and 2a. 
This beautiful species, so characteristic of the Lower Cretaceous strata of the country 
bordering the Upper Missouri, was first described from young specimens collected by Mr. 
Meek and Dr. Hayden at the mouth of Vermillion river, in No. II of their section of the 
Cretaceous rocks. 
Subsequently Dr. Hayden brought from the same region a large series of specimens, exhibiting 
its more mature forms, and in a beautiful state of preservation. 
The younger specimens are accurately represented in the figures given by Hall and Meek, 
(loc. cit.,) and are thus described by them: ‘‘Discoidal depressed; umbilicus wide and shallow; 
volutions four or five, scarcely one-fourth of each embraced in the succeeding one; shell thin; 
surface marked by thirty-eight to forty-five prominent, flexuous, sharp ribs, some of which 
originate in the umbilicus, and others upon the latero-ventral margin, and all extend to the 
dorso-lateral edge, where they bend abruptly forward and terminate before reaching the do: 
line, which is marked by a thin, sharp carina extending to the aperture. Ribs thickened and 
sometimes nodose toward the periphery.’ 
The nodes which ornament the ribs are, as is usual with other species, not developed in the 
very young, and become obsolete on the outer whorls of the very old specimens. 
This shell attained much greater dimensions than would be inferred from the specimens 
figured by Hall and Meek. Some which I obtained are as much as eight or ten inches in 
diameter. On these larger specimens the development of the nodes may be very distinctly 
traced. Until the disk of the shell has reached a diameter of nearly an inch, no nodes are 
observable on the ribs; then a single row begins to appear ; and when the disk is two inches in 
diameter, two rows of nodes are developed, in some specim.-:ns more conspicuous than in others. 
Sometimes a sudden expansion of the ribs near their ventral ends produces even a third row of 
nodes. 
The carination of the dorsal surface, which is continuous in the younger specimens, becomes 
in older ones interrupted, presenting a series of semi-circular lobes, separated by sinuses of — 
similar form. 3 
