132 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



It seems to me, however, from the fact that the type of Lirodiscus does not 

 show the slightest tendency to the prolongation of the anterior muscular scar, 

 so characteristic of this genus, that it cannot be properly included in the 

 same. Its general appearance also seems to indicate nearer relations to 

 Astarte, in which genus both Mr. Conrad and Dr. Lea placed it long back. 



Professor Hall, many years since, proposed the name Paracyclas for a 

 Devonian Lucinoid shell, that may possibly find a place in this genus, either 

 as a distinct section or otherwise ; but, unfortunately, nothing is yet known, 

 I believe, in regard to its hinge and internal characters. 



I have the impression that this genus probably dates back to Palaeozoic 

 times. At' least, we meet with Devonian shells presenting exactly all of the 

 external characters of Lucina. Whether or not they possess the hinge and 

 internal characters of any section of this genus remains to be determined. It 

 is probable, however, that at any rate a" part of them, will be found to present 

 hinge and internal characters warranting their separation under Professor 

 Hall's name Paracyclas; while some of the others may possibly belong to 

 unnamed genera. 



So far as I have observed, there seem to have been no Carboniferous or 

 Permian shells yet discovered showing such close external resemblance to 

 this genus as some of those found in the Devonian ; though, if the group was 

 represented during the Devonian, it must have lived through the Carbon- 

 iferous and Permian periods. In the Triassic and Jurassic rocks, this genus 

 was represented by forms apparently belonging to the typical Lucina section ; 

 which is well represented, along with some species of Cyclas and Myrtea, 

 in the Cretaceous. In the Tertiary, most of the sections are known to occur, 

 and the genus probably attained its greatest development during that period; 

 though it is well represented in the seas of the present epoch, where all 

 of the described sections exist. 



At the present time, this genus has a world-wide geographical distribution, 

 which also seems to have been the case during past geological epochs. 



In regard to the particular name that should be retained for this genus, 

 quite different opinions exist. As Klein, in 1753, used the name Cyclas for 

 a group including a species belonging to one of the subgenera, some have 

 adopted that name for the genus; but as he was not a regular binomial author, 

 and dates before the Linnscan era, I do not see how his name can be adopted 

 for the genus according to rule. H. and A. Adams retain it as a subgeneric 



