181 



They do not assist in collecting food, nor do they aid in effecting any 

 change of place. 



There are three forms of Brachiopod of distinctive habit, apart from the 

 features of their organization, which serve for their distribution into genera. 

 In Terebratula and Rhynchonetta the natural position of the mollusk is to 

 repose upon its back, the dorsal valve is undermost, and a mooring is se- 

 cured by means of a tendinous pedicle let down from the ventral or upper 

 valve, which is beaked over on one side for the purpose. In Crania and 

 Orbicula the position of the mollusk is the limpet position, — the ventral 

 valve is the undermost, and either the pedicle of adhesion passes through 

 it, or the pedicle is dispensed with and the shell itself, like the under valve 

 of Calyptma, is agglutinated to the place of attachment. In Lingula 

 the position of the mollusk is again different : the bivalve elongates its 

 pedicle from the place of attachment to the length of two to six inches 

 and more, and makes its way perpendicularly through mud or sand. More 

 than two-thirds of the Brachiopods belong to genus Terebratula, in which 

 the valves strongly interlock at the hinge-margin, and are held besides by 

 a complicated crossing of adductor and cardinal muscles, in addition to 

 which there are also pedicle and capsular muscles. They inhabit chiefly 

 deep water, and are scattered, somewhat scantily, through all seas. To the 

 geologist the Brachiopods are interesting on account of their great anti- 

 quity. They have existed in every epoch of the world's history. Not 

 quite eighty recent species are known, but of fossil more than eleven hun- 

 dred, and at least three ordinal types of species among the fossil are now 

 extinct. The class may be divided at once into genera. 



Terebratula. Rhynchonella. Crania. 



Orbicdla. Lingula. 



Genus 1. TEREBRATULA, Bruguiere. 



Animal ; attached chiefly by a tendinous pedicle ; arms strongly 

 cirrhated, variously looped and folded, united and fixed to the 

 apophysary skeleton by a membrane, sometimes spiral at their 

 extremities. 



Shell ; inequivalve, equilateral, globosely or triangularly ovate, 

 sometimes smooth, sometimes striated or ribbed, always minutely 

 punctured ; upper or ventral valve beaked over the hinge, the beak 

 being more or less perforated for the passage of a tendinous 

 pedicle, and mostly separated from the hinge by a triangular 

 plate called the deltidium, composed of one or more pieces ; 



