544 Forty-fifth Report on the State Museum. 



produced by a single pair of muscles extending across the ante- 

 lateral region of the visceral area, and by the contraction of 

 these, the valves are closed. The transmedian impressions (*), 

 are made by a triple muscle, one band of which is on one side 

 the visceral area, the other two on the other side, the two lateral 

 components crossing each other in passing from the posterior 

 region 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, laterally, the anterior and posterior extremi- 

 ties of its valves. The muscular region in each valve is sur- 

 rounded by the parietal bands (b), 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 of, and behind these are 

 to be seen radiating vascular markings. 



Shell substance composed of alternating lamella of chitinous 

 and calcareous material. 



Type, Lingula anatina, Lamarck. 



Distribution. (Cambrian?) Lower Silurian — Recent (China, 

 Japan, Guinea, Australia, Philippines, Sandwich Islands.) The 

 genus attained its culmination in the Silurian and Devonian, 

 about seventy species being known from the former on the North 

 American continent. 



Remarks. There are at present no satisfactory means of sepa- 

 rating generically, from the type of the genus, the great majority 

 of fossils passing under the name of Lingula. Internal casts 

 bearing muscular and pallial impressions quite similar to those of 

 recent species have been found in rocks as old as the Lower Silurian 

 (Trenton), and whenever traces of such internal markings are pre- 

 served, there seems to be little difficulty in reconciling them with 

 those of the living shells. Favorably preserved examples from 

 the Carboniferous have shown even the unsymmetrical scars of 

 the lateral or sliding muscles. In many of the extinct forms it 

 is evident that the scars were deeper and the shells thicker and 

 more calcareous than in the species noAV extant. 



The retention of the pedicle in the fossil state has been observed 

 in some Silurian and Carboniferous species. 



96 



