GASTROPODA. 1"'>7 



the t\\" j:ui>- must have ditferent roots. The oldest known shells (Stones River 

 group) that are strictly referable to Cyclonema have relatively coarse revolving 

 ridges. In our Strophostiflus teslilis, on the other hand, the revolving lines are very 

 delicate. 'I' IK- ilitlerence is so marked that we cannot believe that Slrophoslyltts 

 came from the same immediate root as Cyclonema. In our opinion they represent 

 two independent lines. 



The composition of the shell of Ci/clonema must be different from that of the 

 majority of Lower Silurian Gastropoda. On the hills about Cincinnati, where 

 t lii.M-iiiiiU of >p'i'iiiu>iis have been collected, the test is preserved when all the other 

 shells occur as casts of the interior only. Indeed, we have never seen a natural cast 

 i if ' 'i/donema. Another point worthy of notice is the extreme rarity of specimens 

 retaining the apical nucleus. Out of considerably over one thousand good shells 

 before us only six retain the apex entirely. In nearly all of the other cases the 

 evidence at hand seems to show that these minute whorls were lost during the life 

 of the shell, or, at any rate, before fossilization, while in four of the six specimens 

 preserving them they were covered and protected by an encrusting bryozoan. 



The nuclei seen belong to four species, C. bilix var. fluctuatum, C. gracile, C. 

 media It and C. inflatum, the last of which may be but a well-marked variety of the third. 

 In all the nucleus has a glassy appearance, with the first two whorls perfectly smooth, 

 the third with distinct transverse lines only, and all three round and coiled so as to 

 form a blunt apex to the shell. The generic and specific characters begin with the 

 fourth whorl. In C. bilix and C. gracile the whorls decrease very gradually to the 

 first two, but in C. inftatum the third and fourth whorls, with a part of the fifth, are 

 wound into a subcylindrical coil on top of the rapidly expanding succeeding 

 volutions. In C. mediate the conditions may be described as intermediate. Consid- 

 ering the character of the nucleus in these four representative species, it appears 

 that the original stock from which Cyclonema sprang was a low-spired Holopea-\\ke 

 shell.' 



So far. hut a single true Cyclonema is known to us from Minnesota. Other 

 Minnesota shells have been placed in the genus, but in our opinion they do not 

 belong here. In order that the reader may get an adequate and just idea of the 

 genus, and also that the genus may be properly established, we have decided to 

 include a number of species that have not yet been found in the upper Mississippi 

 region. 



Many species have been placed in Cyclonema that do not belong there. Some 

 belongto Gyronema, as for instance, C. percurinatum Hall sp., C. semicurinalum Salter, 

 C. nodulosum Lindstrom and C. carinatum (Sowerby) Lindstrom; others, like C. 



