150 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Yorkia is unknown, but from the character of the casts and the fact 

 that the shells of Micromitra in the same matrix are preserved, it is 

 probable that it was calcareous. The shells of Qiiebecia, Trematobo- 

 lus, and Dearbornia are also calcareous. 



In Kutorgina and Schuchertina, forms that may be referred to 

 either the Atremata or the Protremata, the shells appear to be cal- 

 careous, compact, and without fibrous structure. Messrs. Hall and 

 Clarke, when speaking [1892, p. 174] of the composition of the 

 shell of fossil linguloids, said : "In the group of fossil linguloids, 

 beginning with Lingida, passing through Lingulops and Lingulasma 

 to Trimerella and its allies, there is a regular increase in the relative 

 amount of calcareous matter in the shell, so that the Trimerellas, 

 which are large and ponderous shells, seem to have wholly lost their 

 corneous matter." 



The predominance of corneous or calcareous shell matter does not 

 appear to be of more than generic importance in the classification 

 of the brachiopoda. It is true that the known articulate genera are 

 all calcareous, but it is equally true that among the inarticulate group 

 calcareous shells occur. Alteration, replacement, and removemem 

 of original shell substance have changed the shell of so many species 

 that other characters must be depended upon for classification. 



Microscopic Structure oe the Cambrian Brachiopoda. — In 

 previous work on the Cambrian Brachiopoda, except in the cases 

 above cited, practically no attention has been paid to their micro- 

 scopic shell structure. The importance of this feature in the classi- 

 fication of later species suggested the possible value of a microscopic 

 study of the earlier forms, and at my request Mr. R. S. Bassler, of 

 the United States National Museum, prepared thin-sections and also 

 assisted in the preparation of the accompanying illustrations and in 

 the preliminary study of the sections. 



The preparation of thin-sections of these early brachiopoda is 

 accompanied with difficulties which, together with the lack of suffi- 

 ciently extensive collections, have undoubtedly prevented previous 

 study along the same line. Specimens suitable for sectioning, espe- 

 cially of the calcareous forms, are not at all common, and when they 

 do occur they are almost invariably buried in the rock, and are so 

 thin that the parting of the enclosing matrix does not leave sufficient 

 shell substance for the preparation of sections. In the present work 

 the specific identity of a shell was first determined by uncovering 

 about one-half the valve, and the other half, still embedded in the 

 matrix, was then used in making the section. The structural fea- 

 tures are often restricted to individual lamellae, and the right zone 



