70 TRANSACTIONS OF THE [JaN. 23 



trisoctabedron 2-2 were observed and determined on a few 

 individuals. Instead of tbe ordinary striations parallel to 

 intersections of tbe cube and pyritobedron, tbese crystals bave 

 otbers on tbe faces of tbe octabedron, diploid and pyritobedron 

 and parallel to tbe intersections of tbese witb one anotber. Tbe 

 octabedral faces also sbow striations parallel to tbeir intersec- 

 tions witb tbe cube. 



Tbe paper was discussed by Messrs. Kunz, Kemp, and Braun. 



Dr. MosES next described a mineral from Arizona wbicb 

 seemed to be equivalent to tbe rare sulpbate Ettringite, bitberto 

 only known in minute crystals at Ettring, Switzerland. 



Tbe Arizona mineral resembles pectolite in appearance, but 

 is soft and silky, and occurs in fine, wbite, radiating fibres upon 

 a silicate of tbe same bases. Tbe exact composition bas not 

 been determined, but approximately it may be given as a basic 

 sulpbate of lime and alumina. One analysis recalculated after 

 deduction of gangue sbowed CaO., 26.32, AI2.O3 , 9,85, SO3, 

 18.53, H2O., 45.30. 



Tbe papers are to be publisbed in full in tbe American 

 Journal of Science. 



Mr. L. McI. LuQUER, "On tbe Optical Properties of Cacoxen- 

 ite from various Localities," and " Tbe Optical Properties of 

 Muscovite from tbe Kingsbridge Sbip Canal." 



On Trachosteus and Mylostoma, Notes on their 



Structural Characters. 



BY DR. BASHFORD DEAN. 

 ABSTRACT. 



Tbe discovery of tbe derm plates of tbe entire anterior 

 portion of tbe trunk of Mylodoma makes anotber and important 

 addition to our knowledge of tbe American Artbrodira. 



Tbe material collected from tbe Cleveland sbale at Linton, 

 Obio, by Rev. William Kepler bas recently been added to tbe 

 Museum of Columbia College. In tbe present specimens tbe 

 crusbed plates, as displayed upon tbe slab of cLi}' ironstone, 

 sbow tbat tbe fisb was a small one in tbe group of kindred forms 

 from tbis borizon. It measures from tip of etbmoid to termina- 

 tion of dorsal sbield about fifteen incbes. 



