Characters o/Lingula anatina. 3 



temporary allies have become extinct, the genus may be looked 

 upon as the longest-lived one of its class that is known. Still 

 the remarkable fact is, that its various species do not offer, as in 

 many other cognate genera, any striking variations of form ; 

 indeed it has been stated, though incorrectly, that certain 

 existing species are undistinguishable from, or identical with, 

 others characterizing different geological periods down to the 

 Cambrians *. 



Another subject of interest attached to the study of Lingula 

 is, that throughout its entire existence the shell-substance of 

 its various species has remained constantly and essentially 

 corneous, the mineral, but more subordinate, constituents of 

 this substance being principally phosphate of lime. In 

 another corneous genus, Discina, there are the same paucity 

 of striking test-features, and a role of equal duration. How 

 different with the calcareous genera — for the most part short- 

 lived, and marked with great diversity of specific forms ! 



The great pallial interspace, answering to the general cavity 

 of the shell, is divided into three different compartments, 

 each characterized by special features. 



1. The most important compartment occupies a considerable 

 portion of the posterior half of the shell-cavity, is bounded by 

 a highly muscular wall or parietal band (figs. 1 & 2, b), and 

 contains all the viscera, including the muscles. I propose to 

 name this division the splanchnocoele t, or visceral chamber. 

 With the exception of its frontal portion, which is prolonged 

 in the dorsal valve (fig. 2 ; and fig 3, A), the anterior half of 

 the present compartment is the widest, approaching to nearly 

 the lateral shell-margins. Its posterior half is much reduced in 

 width by a considerable incurvation of the corresponding 

 portions of the wall : and its frontal prolongation causes the 

 anterior outline to differ in the two valves — it being long and 

 tapering in the dorsal, and obtusely rounded, with a slight 

 median point, in the pedicle- or ventral one (fig. 1 ; and fig. 3, B). 

 The differences in the outline of the chamber suggest the 

 propriety of dividing its parietal band into four portions — 

 posterior, post-lateral, ante-lateral, and anterior. 



2. The anterior half of the pallial interspace is open all round 

 (sides and front) except at its back, which is formed by the 

 anterior parietal. It encloses the arms or brachial appendages 



* Mr. Davidson, who has been erroneously credited with this state- 

 ment, has not gone beyond expressing that "many fossil forms resemble 

 in contour such shells as the large L. tumida, L. ovalis, and L. anatina." 

 See Brit. Silurian Brachiopoda, p. 84. 



t From splanclma, internal parts, and ccelia, cavity. 



1* 



