490 FREDERICK W. SARDESON 



continuation of the supposed development through Cambrian time 

 It is unnecessary here to interpret it all in detail. Suffice it to 

 note that in genera like Salpingostoma there is a maturity stage 

 which was anew feature then in gastropod shells. 



It will be useful to determine for the Ordovician species how 

 much less constant they are in their characters than later gas- 

 tropod faunas. There is danger of exaggeration, no doubt, if 

 experience gained from a study of later Gastropoda is applied 

 a priori to these earlier ones, since these may have the shell in a 

 highly plastic stage, and seemingly great differences may not 

 have a high taxonomic value. Such seems to me to be the case 

 in studying large, carefully made collections, and accordingly I 

 am inclined to desire a simplification of the taxonomic plan. In 

 case of Pleurotomaria, both it and Bellerophon may have arisen 

 from Raphistoma species, which argues close linking between 

 these three. Again, Pleurotomaria may be from Raphistoma in 

 part and from Holopea-like species in part; but this argues not 

 only that the dividing up of Pleurotomaria, somewhat as Ulrich 

 and Scofield have done, is logical, if practicable, but also that 

 Raphistoma and Holopea-like species and their consequents are 

 not genetically far apart. In case of Ordovician gastropod 

 species, very excellent specimens are to be had in large numbers, 

 at least in the upper Mississippi valley region, and these, when 

 carefully studied, appear to show a high degree of variability. 

 Ulrich and Scofield [op. cit.) have repeatedly based many taxo- 

 nomic species on a few rather highly variable ones. The Ordo- 

 vician fauna confirms the Cambrian shells to be rightly 

 considered as little diverse, but variable to a high degree as 

 compared with Mesozoic and recent probable descendants from 

 them. 



CONCLUSION. 



The early Cambrian Gastropoda are taken to be in an initial 

 stage of differentiation, the amount of their evolutionary devel- 

 opment at that time being represented by the change from a 

 long curved conical to a short conical, on the one hand, and to 

 short spiral coils on the other. The asymmetrical long conical 

 shell is taken as the most primitive form among Cambrian Gas- 



