SILURIAN FAUNAS. 265 
The maximum diameter of internal casts of this species is about 1 
mm., though specimens with the shell preserved sometimes attain a 
diameter of 1.5 mm. 
Remarks.—As it occurs in New Jersey, this species has been observed 
only in the form of internal casts, but the irregular length of the 
interspaces between the annulations is sufficient to distinguish the 
- species. It has never been observed to occur in such abundance in 
New Jersey as it sometimes does in the Manlius limestone of New 
York. 
CEPHALOPODA. 
ORTHOCERAS sp. undet. 
Plate XXY., Fig. 1. 
A single weathered specimen of a large Orthoceras has been observed 
in the Manlius limestone fauna of New Jersey. It has a maximum 
diameter of about 53 mm. and a total length of 270 mm. The septa 
are rather deeply concave, and in the larger portion of the shell they 
vary from 6 mm. to 9 mm. apart; at the smaller extremity of the 
shell they are sometimes as close together as 2.5 mm. The siphuncle 
is apparently central, and has a diameter of about 2.6 mm. The ex- 
ternal surface characters cannot be determined. 
ARTHROPODA. 
OSTRACODA. 
LEPERDITIA ALTA (Con.). 
Plate XXIV., Figs. 25-28. 
1859. Leperditia alta Hall, Pal. N. Y., vol. IIL., p. 373. 
Description.—Carapace obliquely subreniform, valves stromal con- 
vex, gibbous in the middle, somewhat variable in their proportions. 
Dorsal margin straight, from three-fifths to two-thirds the total length 
of the valves; its extremities obtusely angular. Posterior margin 
