BRACHIOPODA. 5 



of the pedicle-valve to the medic-lateral region of the opposite valve. By the 

 action of these muscles the animal is able to slide apart the anterior and 

 postei'ior 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 lamellse of chitinous and calcareous 

 material. 



Type, Livgula 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 paleozoic 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 palseozoic time, the acuminate type 

 {a), prevailed both numerically and specifically in the earlier faunas {L. acumi- 

 nata, in the Calciferous sandstone ; L. attenuata, in the Llandeilo ; L. crumena, in 



* Among- authors there has been a confusing- variety of nomenclature for the muscuhxr system in 

 LiNGULA. The one that is used here is pi-oposed by Professor William King, and, in its simplicity, has 

 claims for adoption. 



