1899] VooRT>.—Carbo?iiferous Brachiopoda and Mollusca. 79 



case an aperture above the hinge through which the short 

 stalk or peduncle passes. Internally the hinge at the hinder 

 end of the ventral valve consists of two teeth which fit into 

 corresponding sockets in the smaller valve, and enable the 

 animal to open its shell to a limited extent. Attached to the 

 dorsal valve is a beautiful apparatus consisting of thin calcified 

 "processes," which support the tentaculated and ciliated food- 

 procuring " arms." These arms may be flattened parts of the 

 general body-wall, or the outgrowths of it in the shape of 

 long processes which are coiled or twisted in a remarkable 

 way in different genera, and rest upon their calcified supports. 

 In Lingula and Rhynchonella the animal can protrude its arms 

 from the shell. The supports or processes generally form a 

 complicated loop, folded back upon itself, as in the existing 

 Waldheimia (Testicardinate). The loop is reduced to a much 

 more simple form in Rhynchonella, in which the upper part or 

 crura only is developed. In fossil Brachiopods these loops 

 are of great service in classification, but they are often difficult 

 of development. In Spirifcra and other genera common in 

 the Carboniferous limestone, the brachial apparatus reaches a 

 highly complicated form of structure, known as the " spiral- 

 cone type." A brachial apparatus does not exist in the hinge- 

 less section of the Brachiopods (Lingula, Disci?ia, &c.) 



There are several pairs of muscles in a Brachiopod serving to 

 open and close the valves and to control the movements of the 

 spiral arms. As they leave more or less distinct impressions 

 upon the interior surface of the valves they are of importance 

 to the palaeontologist ; they are distinguished by names sugges- 

 tive of their functions ; adductors for closing the valves, divari- 

 cators for opening them, and so on. Vascular markings are 

 also often seen on casts of shells which are suitably preserved. 

 Davidson figures all these structural marks in his splendid 

 work on the British Fossil Brachiop'oda, 1 indispensable to all 

 students of this group. As to their habits, the Brachiopoda 

 are all stationary when adult, but while some attach them- 

 selves, as I have said, others, as Li?igula, form burrows in the 

 sand. Their food consists of minute fragments of animal and 

 vegetable matter, but chiefly diatoms. 



1 Palaecmtographical Society, 1851-1886. 



A 2 



