CYMBIDIUM, A NEW GENUS OF SILURIAN PENTAME- 

 ROID BllACHIOPODS FROM ALASKA 



By Edwix Kirk 

 Of the United States Geological Survey 



In the paleontologic collections made in Southeastern Alaska the 

 upper Silurian pentameroid brachiopods have proved of special in- 

 terest. Typical Conchidiuin is abundant at various liorizons, but of 

 the remaining pentameroids all have been found to represent new 

 genera. I have already described two of these genera — Brooksino and 

 HarpidiuTii. In the present paper I shall describe another genus 

 Cyr/ibidiwn. Of the pentameroids at present ascribed to Conchidiuin 

 there are one or two aberrant types which it may not be possible to 

 place under that genus and for which it may be necessary to create 

 new generic names. The unusual number of new genera, partic- 

 ularly in such a widely distributed group as the pentameroids, is 

 surprising. It is necessary to bear in mind, however, that we are 

 dealing with a boreal fauna which is inadequately known. Further- 

 more, all these new forms occur at a stratigrajDhic horizon scarcely 

 known in areas where extensive paleontologic collections have been 

 made in the past. 



In addition to their biologic interest these pentameroids, owing 

 to their diverse forms and considerable differentiation, have proved 

 of the utmost value in stratigraphic correlations. It is mainly for 

 their use in stratigraphic studies that the forms are being described. 

 It is to be hoped that more extensive collections of these upper 

 Silurian faunas will be made that will permit an adequate treat- 

 ment of the faunas as a whole. 



The faunal sequence in Southeastern Alaska is closely similar to 

 that of England. The lowest Silurian series of which we can be 

 certain at present is characterized in part by graptolite-bearing slates 

 and graywackes which probably correlate closely with the lower 

 Ludlow. These are followed by a great sequence of elastics and thick 

 limestones. In the limestone series the lowest fauna is characterized 

 by a great abundance of Conchidium of the type of knighti^ if indeed 

 knighti itself may not be present. This horizon probably approxi- 



No. 2649.— Proceedings U. S. National Museum, Vol. 69, Art. 23 



3101—26 



