Geology and Mineralogy. 247 



sota, and other notes on the Archaean rocks, and iron ores of other 

 localities. A dozen pages are devoted to further observations on 

 the typical Huronian and on the rocks about Sudbury, Ontario. 

 150 pages are occupied with a review by Prof. Alexander TYin- 

 chell, of the opinions of various American authors " on the older 

 rocks," with the purpose of preparing the way for the settlement 

 of the many mooted questions on the Archaean. The status of 

 of the Huronian is one of them — a topic discussed by the author 

 at length, also in a paper just published by the Geological Society 

 of America. 



4. Remarks on the Perisomic Plates of the Crinoids ; by 

 Charles R. Ketes. (Communicated.) — In an important con- 

 tribution recently made to crinoid morphology* the data have been 

 derived largely from fossil material. This memoir deals chiefly 

 with the structural elements of the ventral covering in several 

 groups of stalked echinoderms ; the " interradial " plates; and 

 the relations these hold to one another and to those of the 

 tegmen. Heretofore some authors have imagined that the disk 

 of paleozoic crinoids was overlaid by a second covering which 

 was not present in the recent forms, but this view is manifestly 

 erroneous. As to the plates between the rays, all writers have 

 discriminated between "calyx" and "disk" interradials, the 

 former term being applied to the massive well-formed plates of the 

 earlier crinoids ; the latter to the small irregular perisomic pieces 

 of the later forms. Contrary to the generally held opinion the 

 investigations of Messrs. Wachsmuth and Springer indicate that 

 all plates between the rays and their subdivisions are parts of the 

 same element and that in geologic times the rigid integument, 

 such as is found in the later Camarata, gradually became evolved 

 from the thinly plated disk of early species. The heavy " inter- 

 radials " are therefore to be regarded as modified perisomic pieces. 



In regard to the ambulacra observations show that these 

 features may be tegminal or subtegminal even among species of 

 the same genus ; and that they are more frequently exposed in 

 the earlier crinoids. From a comparison of young and adult 

 individuals it is further assumed that in the latter forms the 

 ambulacra are often exposed near the margin of the tegument 

 while in other species they are altogether hidden from view. 

 Thus the tegminal ossicles encroach from each side of the am- 

 bulacra and finally close above them, crowding the ambulacrai 

 skeleton inward. 



In the Fistulata all plates between the ambulacra, and between 

 radials and orals are perisomic. In most of the species the ven- 

 tral sac is porous, while in others there is anterior to the inflation 

 a perforated plate like the madreporite of the urchins. The 

 Larviformia are now restricted to forms having the ventral 

 surface composed of orals only, with no perisomic plates. 



* Perisomic Plates of the Crinoids. By Charles Wachsmuth and Frank 

 Springer. Proc. Acad. Nat. Sci. Phila., 1890, pp. 345-392, with two plates. 



