234 SMITHSONIAN MISCFXLANEOUS COLLECTIONS VOL. 53 



three very short rudimentary segments and a plate-like pygidium 

 beneath the great spine on the fifteenth segment." These specimens 

 convinced me that Olenelliis thonipsoni passed through a Mcsonacis 

 stage before becoming a typical OlcncUits. I put the specimens away 

 in hopes that others would be found throwing more light on the 

 problem. In the collection made by Dr. Charles Schuchert at York 

 in 1896 an otherwise typical form of Olencllus tliompsoni 40 mm. 

 in length was found to have four rudimentary segments and a py- 

 gidium beneath the spine on the fifteenth segment, but it was not 

 until 1909 when Prof. Atreus Wanner sent me a large series of 

 specimens showing young stages of growth, also adults with from 

 two to four rudimentary segments posterior to the fifteenth spine 

 bearing segment that sufficient material was available to definitely 

 conclude that Olenellus thompsoni passed through a Holmia, a Mes- 

 onacis, and a Pcsdeumias stage of development, and later became a 

 typical 0. thompsoni with a terminal telson by the absorption of 

 certain rudimentary segments and a plate-like pygidium. 



FUTURE WORK 



It is exceedingly desirable that more collecting should be done 

 in the Lower Cambrian formations of the Reval region of Russia ; 

 in Finland; and in the various localities in Sweden, Norway, and 

 England. I am sure that important information in relation to the 

 Mesonacidse would be secured by a systematic search for more and 

 better material. In America we will continue the work in 1910 in 

 western Newfoundland and the Straits of Belle Isle, and in British 

 Columbia and Alberta, and another season will be spent in Nevada 

 and California. 



ACKNOWLEDGMENTS 



I am greatly indebted to Prof. Atreus Wanner, Superintendent 

 of Public Schools at York, Pennsylvania, for very generously per- 

 mitting me to study and illustrate the material in his collection.' 

 Prof. H. Justin Roddy, of the State Normal School at Lancaster, 

 Pennsylvania, permitted me to examine the collection he had made 

 in Lancaster County, and loaned me specimens, and both he and 

 Professor Wanner took me over the areas from which they collected 

 specimens in central Pennsylvania. Dr. Job. Chr. Moberg, of the 



^ This form is now included in Pccdcuviias transitaiis. 



" Professor Wanner has since presented to the United States National 

 Museum the specimens illustrated and described in this paper. 



