2 PROCEEDINGS OF TPIE NATIONAL MUSEUM vol. 69 



mates closely to the Aymestry limestone of the middle Ludlow. In 

 England the upper Ludlow and Downtonian apparently do not carry 

 large normal marine faunas. In Alaska, however, these higher lime- 

 stones are richly fossiliferous, and it is from them that these new 

 pentameroid genera have been collected. In the interior of North 

 America we can find a stratigraphically comparable fauna perhaps 

 onl}'^ in the Monroe group of Michigan. This group has been ascribed 

 in part to the Devonian b}^ some geologists. For the present it 

 would appear that the Monroe should be retained in the Silurian. 

 The so-called Devonian elements in the upper Monroe fauna are 

 far from convincing. In the boreal upper Silurian are many fossils 

 that judged by eastern American standards would be considered 

 Devonian. It is indeed a matter of considerable difficulty at times 

 accurately to separate upper Silurian and Middle Devonian faunas 

 from Alaska when small collections of fossils are available. This 

 applies particularly to the corals. Even in the case of crinoids, 

 however, which are unusually diagnostic, I have found such Euro- 

 pean Devonian forms as Hexacrinus and Codiacrinus in the Brook- 

 sina zone of the upper Silurian of Southeastern Alaska. To be sure, 

 dorsal cups alone represent these genera, and if complete specimens 

 were available they might prove generically distinct from their 

 Devonian relatives. Again, in the Ural Mountains we apparently 

 have an admixture of Silurian and Devonian types. Conchidiuin 

 is supposed to range upward into the Devonian. I think that here 

 again we have to deal with boreal Silurian faunas, certain con- 

 stituents of which are mainly known to us in eastern America and 

 western Europe only in Devonian time. The solution of the prob- 

 lem will probably be had only when the boreal faunas are recognized 

 as such, their sequence established, and their southward invasions 

 recognized. 



CYMBIDIUM, new genus 



This genus is represented by two known species from the upper 

 Silurian of Southeastern Alaska. Cymbidium acutum, new species, 

 which has been chosen as the type, is fairly abundant in the lime- 

 stones to the east of Edna Bay, on the south shore of Kosciusko 

 Island. In a couple of hours' collecting one specimen with both 

 valves preserved and a dozen or more dissociated valves were found. 

 Near this locality and from a lower horizon, associated with Brook- 

 sina alaskensift Kirk, another species was found. 



Superficially Cymbidium^ though clearly a pentameroid, strongly 

 suggests the general habit of an orthoid. In the type species the 

 valves are almost equally convex, though the produced beak of the 

 pedicle valve makes this the larger. The shell is wider than long, 

 and the hinge line is about one-half the maximum breadth of the 

 shell. In size Cymbidium falls with Gypidula and the allied genera. 



