HARPIDIUM, A NEW PENTAMEROID BRACHIOPOD 

 GENUS FROM SOUTHEASTERN ALASKA. 



By Edwin Kirk 



Of the United States Geological Survey 



In an earlier paper^ I described the new genus Brooksma from a 

 series of sediments in southeastern Alaska tentatively referred to 

 the upper Silurian. In that paper will be found a brief discussion 

 of the faunal affinities of the series. A more detailed study of the 

 Brooksina fauna together with the famias of lower and liighei 

 horizons strengthens the belief that the formations in question 

 should be assigned to the upper Silurian rather than to the lower 

 Devonian. There are certain elements suggestive of the Devonian, 

 but there is little to compel a correlation with the widespread and 

 characteristic lower Devonian or Helderbergian faunas. On tlie 

 other hand, our scant laiowledge of latest Silurian normal marine 

 sediments v»'ith their contained faunas leaves a faunal gap into 

 which these Alaskan faunas may well fit. Within this time interval 

 should be placed, I think, those faunas of the Ural Mountains com- 

 monly referred to the lower Devonian. 



In the limestone series characterized by Brooksina alaskensis is 

 another pentameroid of considerable interest. To this genus the 

 name Harpidtum is here given. The genus is not represented else- 

 where than in this restricted zone of the upper Silurian of South- 

 eastern Alaska, so far as IniOAvn. Conchidium {?) occidentalis Hall 

 from the Guelph dolomites of Ontario resembles Harpidium in 

 general form. Detailed knowledge of the structure of the species 

 is wanting. G. (.<^) occidentalis has, however, obscure radial plica- 

 tions that are wanting in the Alaskan species of Harpidium. Even 

 so its affinities may prove to be closer to Ilarpidiuin than to Conchi- 

 dium. to which it was doubtfully referred by Hall and Clarke. 



GENERIC DIAGNOSIS 



Harpidium may briefly be defined as a nonplicated pentameroid 

 of large size with highly arched valves. The pedicle valve has a 



' Kirk, K.. Brooksina, a new peutarueroid genus from the upper Silurian of southoast- 

 em Alaska : Pioc. U. S. Nat. Mus., vol. 60, art. 19, pp. 1-8, pi. 1, 1922. 



No. 2569.— Proceedings U. S. National Museum, Vol. 66, Art. 32. 



12019—24 1 



