GEOLOGY AND MINERALOGY. 159 



genera this framework is wanting. The coiling of the arms— the mode of which 

 varies in the diffarent families — gives a larger amount of surface, than would 

 otherwise result, for the development of the cilia (or ciliated cirri) with which 

 they are clothed. In the living species unprovided with internal shelly processes, 

 they are more or less extensible, and this was probably the case also with regard 

 to the extinct forms similarly constituted. In the species, however, in which 

 these shelly processes were present, the arms were undoubtedly incapable of 

 protrusion beyond the shell. Besides which, the mode of articulation of the 

 valves, in the generality of cases, is such as to prevent the opening of the shell to 



any extent. The shell itself — equilateral in form, but with unequal valves is 



traversed in most genera by minute pores or tubular prolongations probably con- 

 nected with respiratory functions, although in the family of the RhynconellidcB, as 

 well as in certain spirifers, &o., these pores are not observable. The shell is then 

 said to be impunctate. The fossil brachiopods were attached to sub-marine 

 objects by a predicel or byssus passing thi'ough an opening in one of the valves— 

 or, in some genera, by the direct surface of the larger valve; otherwise they 

 existed as free shells. Whether fixed or free, however, the brachiopods in the 

 adult condition possess no powers of locomotion. 



The Brachiopoda admit of being grouped in two sub-orders : 1. Brachiopoda 

 proper ; and, 2- Rudistes. There is at present a strong disposition to class the 

 latter with tlie Lamellibrauchiata, placing them near the Chamidse : a view sup- 

 ported especially by Deshayes, Davidson, Bayle, and "Woodward, in opposition to 

 the opinion of Goldfuss, d'Orbigny, Pictet, McCoy, Philippi, and other observers. 

 The question is still an open one — perhaps eventually to be settled by raising the 

 Rudistes to the rank of a distinct order intermediate between the true brachiopoda 

 and the ordinary acephala. The perforated shell-structure, (although not in all 

 cases detected), the conformation of the hinge, and the mode of growth of the 

 shell, are among the more salient characters which separate the Rudistes from 

 these latter. Some very able papers, by Bayle and others, taking the lamelli- 

 branchiate view, may be seen in the thirteenth and fourteenth volumes of the 

 Bulletin de la Societe Geologique de France. In the annexed distribution the 

 Brachiopoda proper are alone considered. 



Table I. — Families of the Brachiopoda. 



No true / ^j^^^^ _ ^^^.^ ^^ ( Neither valve perforated Lingulid*. 



articulation \ ' ' ' ^ Slit or Poramen in oiifi valve Discinidee. 



by hinge- j gj^^j^._ calcareous, \ f ''^ ^^"tj"^' or obscure Crauiad^e. 



teeth, f I Area largely developed Calceolidee. 



/Nointernal shelly process,^ Hinge-line straight Orthisidae. 



Shell containing calcareous spiral 



processes pointing outwards Spiriferidse, 



Slight internal process ; hinge-line 

 Valves j ^ arched ; no area. Arms in living 



articulated / _ , , , ,, / type, pointing downwards and 



i.,T i-.i,,o.„ \ Internal shellyprocesses, ( . ' _., 



toyhmge- \ ji >\ m^ards Ehynconellidje. 



tppth * I I 



' ' Internal shelly loop; shell punctate, 



thin Terebratulidee. 



Internal complicated processes ; 



shell thick Thecidese. 



In the Productus type of the Orthisidse this character is sometimes obscure. 



