BRACHIOPODA. 



17 



strated by a thin transverse section, which 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 

 shell-substance is essentially corneous, and the mineral 

 intermixture is calcic phosphate with traces of calcic 

 and magnesic carbonates. 



The organic and inorganic constituents are arranged 

 in alternating lamellae, the former, according to Gra- 

 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 

 strong punctae, visible to the naked eye on the internal 

 surface, where, according to the author above cited, the 

 calcareous layers of the test are thickest. The same 

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

 determined species from the Black shale of Madison count}', 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 canals 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 



* Journal de Conchyliologie, 2d ser., vol. viii, p. 59. 1S60. 



Fig. 13. Magnifleil section of 

 the thickest portion of the 

 Shell o'iLingula anatina (from 



DAVIDSON.arteiGRATIOLET). 



A^ corneous la3'ers. 

 B, niiuercil layers with ver- 

 tical canals. 



