490 



FORTT-FIFTB REPORT ON TBE STATE MUSEUM. 



usually has a pitted surface. In Scfizophoria there are four or 

 more subparallel longitudinal main pallial sinuses which 

 subdivide toward their distal extremities. (Plate 11, figs. 

 21-23.) 



Further reference will be made to the structure of the sinuses 

 and genital organs. 



Structure of the Test. 



The valves of the Brachiopoda are composed of successive 

 laminae of heterogeneous structure. These may be wholly 

 calcareous, and not more than two or three in number, or 

 alternately calcareous and corneous, and not restricted to any 

 numerical limit. Shells largely corneous or chitinous in their 

 composition are restricted to the inarticulate division, but the 

 inarticulates do not all possess corneous shells. In the living 

 Lingula there is an alternation of corneous and calcareous layers 



varying in thickness, the former being 

 compact and imperforate, the latter 

 fibrous or prismatic and crossed by a 

 great number of minute tubules. In 

 fossils of this group the calcareous 

 matter is frequently more or less 

 removed, so that shells in this con- 

 dition may appear to be essentially 

 corneous in their composition. In the 

 group of fossil linguloids, beginning 

 with Lingula, passing through Lin- 

 gulops and Lingulasma to Trimerella 

 Y l«^^™trZ^^ its allies, there is a regular increase 



i^M^i^SSS^ 111 the relative amount of calcareous 

 ticai canals. (Gratiolbt.) matter in the shell, so that the Tri- 



merellas, which are large and ponderous shells, seem to have 



wholly lost their corneous matter. In Crania also, the shell is 



essentially calcareous and the successive layers are punctured by 



tubules or vertical canals, which are largest at their openings on 



the interior of the valves, and narrow toward the outer surface. 



These never pierce the thin outer or epidermal layer of the 



shell. 



42 



