806 THE PALEONTOLOGY OF MINNESOTA. 



[Cyrtooeras billingsi. 



The undulations of the surface lamella) 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 revolving ridges. 



Formation and locality. ID the Trenton limestone at Janesville, Wisconsin, and in the shales at 

 St. Anthony Park, Minneapolis. 



CYRTOCERAS BILLINGSI Salter, 1851). 



PLATE LX, FIS. 10. 



Cyrtoceras billingsi SALTEK, 1859. Figures and Descr. Canad. Organ. Rem. Decade 1, p. 33, pi. vn, 

 flg. 6 (non 5). 



Salter, in proposing, in the work cited, to rechristen the species C. lamelloxian 

 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. halliannm 

 therefor. Mr. Salter also figured as C. billingsi 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 laraellose 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) is a synonym for C. 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 8 mm. 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 lamella:, 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 

 lamella- 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 



