22 NOTES ON THE BEITISH JURASSIC BRACHIOPODA. 



to be equilateral. Thej are not, however, equivalve ; one 

 of the valves being usually larger than the other. In this 

 respect they difi.er from the Pelecypoda or true bivalves, 

 which are usually equivalve, but inequilateral. 



The valves are either articulated by means of two curved 

 teeth developed from the margin of the ventral valve, which 

 St into corresponding sockets in the smaller valve, or they 

 are inarticulated, in which case the valves are maintained 

 in position by muscular action alone. 



The inarticulated brachiopods belong to the group Lyopo- 

 mata of Owen, or the Tretenterata of King; while the arti- 

 culated belong to the Clisterentata of King, which is 

 equivalent to the Arthropomata of Owen.* 



The large valve is usually called the ventral valve, the 

 small one the dorsal valve. Ofcher names have been given, 

 and various reasons assigned for so doing ; but ventral and 

 dorsal are the two best adapted for our purpose. 



The ventral valve in many of the genera, especially in the 

 Terebratulidge, has a more or less pronounced beak, which is 

 generally perforated at or near its extremity by a circular 

 foramen, which serves for the emission of a bundle of 

 muscular fibres called the peduncle, by which the animal 

 forming a byssus at one end attaches itself to some sub- 

 marine object. 



* This division for all practical purposes will serve us. We must not, 

 however, forget that a more modern grouping of the Brachiopoda is now 

 adopted by many of our eminent palaeontologists. It is as follows : — 

 I. Atremata. Examples: Paterina, Obolm, Trivierella, Lingulella, 



Lingula, etc. 

 II. Neotremata. Examples : Crania, Discina, Discinisca, etc. 



III. Protremata. Examples : Orthis, Strophomena, Leptana, Cho- 



netes, Productus, Thecidea, etc. 



IV. Telotremata. Examples : Atnjpa, Gyrtia, Spirifer, Spiriferina, 



Bhynchonella, Terebratula, Terebratulina, Magellania, etc. 

 (6, pp. 65-76). 



