THE BRACHIOPOD SHELL. 297 



Among the calcareous, inarticulate brachiopods the shell of the Cambrian genus Oholella 

 shows a dense, compact, slightly lamellated structure made up of a granular groundmass pierced 

 by extremely small tubules or pores. The substance of the shells of Rustella and Yorkia is 

 unknown, but from the character of the casts and the fact that the corneous shells of ilicromitra 

 in the same matrix are preserved it is probable that it was calcareous. The shells of Queiecia, 

 Trematoholus, and Dearhomia are also calcareous. 



In Kutorgina and ScJiuchertina, forms that may be referred to either articulate or inarticu- 

 late genera, the shells appear to be calcareous, compact, and without fibrous structure. 

 Hall and Clarke, when speaking [1892a, p. 174] of the composition of the shell of the fossil 

 linguloids, say: 



In the group of fossil linguloids, beginning with Lingula, 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 brachiopods. It is true that the known 

 articulate genera are all calcareous, but it is equally true that among the inarticulate group calca- 

 reous shells occur. Alteration, replacement, and removal of original shell substance have changed 

 the shell of so many species that other characters must be depended on for classification. 



IdCROSCOPIC STRTJCTTJRE. 



In previous work on the Cambrian Brachiopoda, except in the cases above cited, practically 

 no attention was paid to the microscopic shell structure. The importance of this feature in 

 the classification of later species suggested the possible value of a microscopic study of the 

 earlier forms, and at my request Mr. E. O. Ulrich and Mr. R. S. Bassler prepared thin sections 

 and also assisted in the preparation of figures 1-15 (pp. 298 and 299) and in the preliminary 

 study of the sections. 



The preparation of thin sections of these early brachiopods is accompanied with difficulties 

 which, together with the lack of sufficiently extensive collections, have undoubtedly prevented 

 previous study along the same line. Specimens suitable for sectioning, especiallj^ of the cal- 

 careous 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 inclosmg 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 features are often 

 restricted to individual lamellae and the right zone for microscopic examination was deter- 

 mined simply by close observation as grinding proceeded. Both vertical and tangential sec- 

 tions were prepared, the former cuttmg the shell at right angles and the latter cutting the shell 

 in planes more or less parallel to the layers or lamellfe of which it is composed. The most inter- 

 esting results were obtained from the tangential sections, as the thin shells showed little decided 

 structure in vertical sections. 



