4H 



HISTORICAL GEOLOGY 



Mollus<x>idea 



Brachiopods. — This great class was especially important in the 

 Paleozoic, not only because of the abundance of individuals, but also 

 because certain species, though prolific, were short-lived, being abun- 

 dant in one period or a subdivision of one period and becoming ex- 

 tinct at its close. As a result, when the fossil remains of such species 

 are found in a stratum, proof is offered of the age of the formation. 

 Brachiopods or Lamp Shells (so-called because of the resemblance of 

 some of them (Fig. 384 A, F) to Roman lamps) are inclosed by 

 two shells or valves and can usually be readily distinguished from 



Fig. 384. — Cambrian brachiopods: A, Billingsella coloradoensis ; B, Lingulepis 

 acuminata; C, Obolella atlantica; D> Acrothele subsidua; E, Micromitra bella; F, 

 Kutorgina cingulata. 



other shellfish by two characteristic features : (1) the bilateral sym- 

 metry of their shells, i.e., a line drawn from the beak to the front di- 

 vides them into equal parts; and (2), in most cases, by the dissimi- 

 larity and unequal size of the two valves. The name brachiopod 

 (Greek, brachion, arm, and pous, foot) refers to the two long spiral 

 " arms " inclosed between the valves by means of which food is ob- 

 tained and respiration carried on. These " arms " are attached to a 

 shelly apparatus, sometimes in the form of loops and sometimes in 

 spirals (Fig. 403 M, p. 432). 



Brachiopods are divided into two great subdivisions : the hingeless 

 (Fig. 384 B, D, E) or inarticulate, with phosphate of lime shells the 

 two valves of which were held together only by the muscles of the 

 animal ; and the- hinged (Fig. 384 A, C) or articulate, with calcareous 



