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 IY 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 IY k (fig. 19), an un¬ 
determined species from the Black shale of Madison 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 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. 12. Magnified section of 
the thickest portion of the 
Shell of Lingula anatina (from 
Davidson, afterGEATiOLET). 
A, corneous layers. 
B, mineral layers with ver¬ 
tical canals. 
