SPONGES, GRAPTOLITES, CORALS. 93 



Streptelasma.] 



This species seems to be near S. corniculum Hall, but is readily distinguished by 

 its smaller size, more rapidly expanding corallum, more distinct fovese, and less 

 twisted as well as less laminar septa. S. profundum Conrad (Hall) is straighter, has 

 a deeper calyx, and is entirely without the central twisting or inosculation of the 



septa. 



Formation and locality. Rather rare at the top of the Trenton limestone, near Fountain, Minnesota. 

 Ten specimens collected by the author are now in his cabinet. 



STREPTELASMA RUSTICUM Billings. 



PLATE G, FIGS. 22, 2a. 



1851. Streptelasma corniculum EDWARDS and HAIMB (non Hall). Monograph Poly. Foss. des Terr. 



Pal., pi. 7, flg. 4. 



1858. Petraia rustica BILLINGS. Geological Survey of Canada ; Report of Progress for 1857, p. 168. 

 1875. Streptelasma corniculum NICHOLSON (non Hall). Palaeontology of Ohio, vol. ii, p. 208. 

 1875. Streptelasma corniculum (partirn) NICHOLSON. Palaeontology of the Province of Ontario, p. 26. 

 1882. Streptelasma corniculum HALL. Eleventh Report State Geologist of Indiana, p. 376, pi. 51. 



figs. 2-4. 



1889. Streptelasma rusticum MILLER. North American Geology and Palaeontology, p. 205. 

 1889. Streptelasma corniculum NICHOLSON. Manual of Palaeontology, vol. i, p. 247, flg. 127B; p. 278, 



279, figs. 156A, B; p. 280, flg. 157; p. 297, flgs. 178a, 178b. 



Original description.-" Straight or slightly curved, covered with a strong epitheca, 

 which is more or less annulated with broad shallow undulations ; radiating septa 

 about one hundred or usually a little more ; much confused in the center, where they 

 form a vesicular mass ; every alternate septum much smaller than the others, only 

 half of the whole number reaching the center. Length from two inches and a half 

 to three inches and a half. Diameter of cup one inch to one inch and a half; depth 

 of cup half an inch or somewhat more." 



This species attains a larger size than S. corniculum Hall, with which it is usually 

 identified, and differs from it in having the septa more strongly twisted and coarser 

 in approaching the center of the calyx, where they form a vesicular mass or pseudo- 

 columella. This central twisting of the septa is not so pronounced in Minnesota 

 specimens as it is in individuals from Richmond, Indiana. A large series of speci- 

 mens will exhibit considerable variation in the extent of the central vesicular mass, 

 and though it is usually of much coarser construction in the form described by 

 Billings, it cannot be denied that the two species are very closely related. 



Formation and locality. In the Hudson River group near Granger, and at Spring Valley, Minnesota; 

 Graf, Iowa ; Richmond, Indiana ; Oxford and Dayton, Ohio. Snake island, lake St. John, and Manitou- 

 waning, Georgian bay, Canada. 



Collectors. W. H. Schofleld, E. O. Ulrich, and the writers. 

 Mus. Reg. Nos. 7753, 7754. 



