BRACHIOPODA. 
5 
of the pedicle-valve to the medio-lateral region of the opposite valve. By the 
action of these muscles the animal is able to slide apart the anterior and 
posterior extremities of its valves* The muscular region in each valve is sur¬ 
rounded by the parietal bands (6), which leave more or less distinct impressions 
upon the shell. 
The anterior internal surface of each valve bears traces of two strong pallial 
sinuses, which nearly meet in the axial line before reaching the anterior margin. 
In front and behind are radiating vascular markings. 
Shell substance composed of alternating lamellae of chitinous and calcareous 
material. 
Type, Lingula anatina, Lamarck. 
Observations. Few of the larger genera of the brachiopoda form so integral 
a group and present so well defined limitations as the genus Lingula. Not¬ 
withstanding, however, the compactness of the genus, the discrimination of 
palaeozoic species is frequently attended by very great embarrassments, arising 
from the tenuity of the shell, its liability to distortion in process of fossilization, 
the general similarity in the matter of external ornamentation, and the usual 
obliteration of the definition of the muscular scars by maceration or from the 
character of the matrix. 
Primarily, the shells of palaeozoic Lingulas present two extremes of varia¬ 
tion in outline, ( a ) an acuminate or sub-triangular form, in which the rostral 
area is very narrow, with long, sloping post-lateral margins, and a rounded or 
transverse anterior margin; ( b ) a sub-quadrate form, with the posterior or 
rostral margins converging toward the beak. The passage-forms between these 
extremes present outlines varying from ovate to elliptical, and here the great 
majority of species is to be assigned. While all these variations in form have 
maintained a contemporaneous existence in palaeozoic time, the acuminate type 
(a), prevailed both numerically and specifically in the earlier faunas ( L . acumi¬ 
nate s, in the Calciferous sandstone; L. attenuata , in the Llandeilo; L. crumena , in 
* Among- authors there has been a confusing variety of nomenclature for the muscular system in 
Lingula. The one that is used here is proposed by Professor William King, and, in its simplicity, has 
claims for adoption. 
