BRACHIOPODA. 1 7 



strated by a thin traiisver.se section, whicli shows them to be simply opaque 

 masses without evidence of structure save a thin transparent external cover- 

 ing. Enlarged figures of this internal cast and of the ovoid bodies are given 

 upon Plate IV k. 



On account of the extreme tenuity of the shell in the fossil Lingulas, it is 



difficult to ascertain its structure from .sections. The ^^^^ T'^'ri' a, 



shell-substance is essentially corneous, and the mineral ^ jp— ^ 



intermixture is calcic phosphate with traces of calcic 

 and magnesic carbonates. ^ 



The organic and inorganic constituents are arranged " 4 



in alternating lamellse, the former, according to Gr.a- -— ^ ^ ^^ 



TIOLET,* having a fibrous structure, and the latter being 

 traversed by numerous microscopic canals. A finely ' 



preserved specimen of an undetermined species from 

 the Waverly sandstone at Pierrepont, Ohio, shows vn^.ii. M.ij:iuiiea sccuon of 



the lliickest )ioi'tion of the 



stronf punctiB, visible to the naked eye on the internal sheii or /.ih^kIh «»««)!« dvom 



^ ^ " Davidson, aflertiKATioLET). 



surface, where, according to the author above cited, the ^, .■..incous laycs. 



7>. niiner.Hl layers with vcr- 



calcareous layers of the test are thickest. The same ticaic-iiuiu. 



character is also seen in the specimen figured on Plate IV k (fig. 19), an un- 

 determined species from the Black shale of MadLson county, Kentucky. The 

 laminae of the shell have exfoliated, exposing the internal cast, which is covered 

 with minute papillae, apparently the fillings of the inner openings of these 

 tubes. It would, therefore, seem that these vertical cumals have sometimes 

 attained a greater development in the extinct than in the living species. 



The surface ornamentation is subject to but little variation, usually con- 

 sisting of concentric lines marking the successive stages in the growth of 

 the shell. From the paucity of external characters, arises much of the diffi- 

 culty attending the determination of species, similar in their general aspects. 

 Still, a few species, exceptions to the rule, have a striking shell-orna- 

 mentation. In L. punctata, of the Hamilton group, the fine surface lines are 



* Jouniiil (le Ccmchyliologie, 2d ser., vol. viii, p. 59. 1860. 



