S06 THE PALEONTOLOGY OF MINNESOTA. 
[Cyrtoceras billingsi. 
The undulations of the surface lamelle consist of numerous small festoons 
caught up at regular intervals on the successive growth-lines. Over the dorsal and 
ventral surfaces are traces of low revolvimg ridges. 
Formation and locality.—In the Trenton limestone at Janesville, Wisconsin, and in the shales at 
St. Anthony Park, Minneapolis. 
CYRTOCERAS BILLINGSI Salter, 1S59. 
PLATE LX, FIG. 10. 
Cyrtoceras billingsi SALTER, 1859. Figures and Descr. Canad. Organ. Rem. Decade 1, p. 33, pl. vu, 
fig. 6 (non 5). 
Salter, in proposing, in the work cited, to rechristen the species C. lamellosum 
Hall, with the name C. billingsi, overlooked the fact that d’Orbigny had recognized 
the preoccupancy of that term, and in 1850 had introduced the name C. hallianum 
therefor. Mr. Salter also figured as C. billingst two specimens, one of which shows 
the form and rate of expansion of the shell as we have above given it, and also the 
peculiar festooned, squamous growth-lines and faint longitudinal ridges of the 
surface. The other of his figures (fig. 6) represents a more rapidly expanding shell, 
with lamellose growth-lines which are simple and not festooned, and are strongly 
retrose on the venter. By finding both forms represented in the Silurian rocks of 
Minnesota, retaining all the features exemplified in each of Salter’s figures, we are 
convinced that there are here two quite distinct species. As Salter’s name, so far 
as it is based on the first of his figures (fig. 5) isa synonym for CU. hallianum the 
term C. billingsi may properly be employed for shells conforming to the type of the 
second of his figures. 
One excellent specimen and fragments of others permit the following descrip- 
tion of this species. 
Shell very arcuate and rapidly expanding. A specimen measuring 57 mm. 
in length on the ventral periphery and 32 mm. on the dorsal, has an apertural 
diameter of 23 mm. dorso-ventrally and a posterior diameter of 8mm. The arc 
traversed in this length is approximately one-third of a volution. Transverse 
section nearly circular, venter very broad. Surface covered with fine, crowded, 
subequidistant lamelle, from .3 to .5 mm. apart; these are projected forward or 
toward the aperture and may be sufficiently long to overlap each other. The 
interspaces become somewhat greater toward the aperture. On the venter these 
lamelle make a short, decided curve backward. The form of the septa and course 
of the sutures are not known. 
The external characters of this species are such as to readily distinguish it. 
The single respect in which a difference from the specimen described by Salter 
