268 MINERALS AND GEOLOGY 



of a pedicel, and is sometimes covered by a " pseudo-del tidium." 

 The hinge-line in some genera is straight or horizontal (orthis, stro- 

 phomena, spirifer, <&c.), and in others curved (rhynconella, terebratula, 

 <&c.). The outer surface of the shell may be smooth, or (and more 

 commonly) it may be marked by fine or coarse radiating lines. In 

 many genera the centre of the ventral valve (more rarely the dorsal) 

 shows a depression or " sinus," and there is a corresponding elevation 

 or " mesial fold " on the other valve. The substance of the shell 

 may be " punctate " or " impunctate " i.e. traversed, or not, by 

 minute pores ; but this character often becomes inconspicuous through 

 fossilization. 



The arms or tentacles in the interior of the shell are always 

 attached to the smaller valve. In some genera (orthis, &c.\ these 

 are without supporting calcareous structures. In other genera (rhyn- 

 conella, <c.) they are attached to two very short processes or " crura." 

 In others (spirifer, <c.), they are wound around two calcareous 

 spires. In others, again, (terabratela, waldheimia, &c.) they are 

 supported by a short or long calcareous loop. Unfortunately, these 

 internal structures in fossil examples are often not directly observ- 

 able. As a rule, the shell must be carefully ground down with fine 

 emery on an iron plate, or artificial sections must be prepared for 

 their recognition. In like manner, the muscular impressions on 

 the inner surface of the valves are only occasionally observable. 

 These impressions differ in shape and mode of arrangement, but are 

 usually in pairs ; four to six in number. 



The brachiopods are classed under two principal divisions. In one 

 (Articulata of Huxley, Arthropomata of Owen, Testicardines of 

 Brown, Clistenterata of King, Apygia of Zittel) the valves have a 

 more or less distinct hinge or interlocking teeth, and (as seen in 

 the living forms) the intestine is without any anal opening. In the 

 other (Inarticulata of Huxley, Lyopomata of Owen, Ecar -dines of 

 Brown, Tretenterata of King, Pleuropygia of Zittel), there is no 

 proper hinge, and the intestine terminates in an anal oiifice at the 

 side of the body. 



Division 1 Articulata or Clistenterata : Brachiopods with hinged 

 shell. In living forms, the intestine closed : 



This division includes (principally) the following families : 1, 



