54 PALEONTOLOGY. 



Class I.— BRACHIOPODA. :: 



The mollusks of this class are so called because the chief 

 moving and prehensile parts (fig. 1 1 , d, f), resemble the " arms " 

 of some polypes : they are spirally disposed, fringed and 

 ciliated, and may answer to those of the Bryozoa, or " cilio- 

 brachiate" polypes. The soft parts are protected by a shell 

 consisting of two valves, one (ib. d) applied to the dorsal, the 

 other (v) to the ventral surface. The latter has a prominent 

 notched or perforated beak, through which in most, a pedicle 



(n) passes, to attach the animal to 

 some foreign body. There are several 

 pairs of muscles (o, p, q) for opening 

 and shutting the shell. This, in the 

 first order of the class (Arthropo- 

 mata)^ in which the shell-valves 

 Fig, ii. are articulated together, has more 



Waldheimiaflavescens. or less tlie slia P e of an ancient 

 roman lamp. 



The lamp-shells, more than any other group, have suffered 

 with the lapse of time. Of 1300 known species, only 75 are 

 living ; and of the 34 genera, the larger part (21) are extinct. 

 The number of generic forms is greatest in the Devonian 

 period and least in the upper oolites, after which a second set 

 of new types gradually appears. The preponderance of fossil 

 Brachiopoda is contrasted with the scarcity of the recent shells 

 even more strongly by the abundance of individuals than by 

 the number of species ; for the living shells mostly inhabit 

 deep water and rocky situations inaccessible to the dredger, 

 and are seldom obtained in large numbers. 



The genus Terebratula, as now restricted to shells with a 

 short internal loop, musters above 1 00 fossil species, of which 



* Gr. brachys, an arm ; pons, a foot. 

 f Gr, arthros, a joint; poma, a valve or lid. 



