192 AN INTRODUCTION TO THE STUDY OF FOSSILS 



apparent ill effects. Crania will also thrive under most adverse 

 conditions. Such adaptability is probably the cause of the 

 persistence of these two genera from Ordovician times to the 

 present. 



The persistence of genera and species in time varies greatly. 

 Some genera, as Hipparionyx and Rensselaeria (Lower Devo- 

 nian), survived but a short time ; while others, as Lingula and 

 Crania, have existed since Ordovician times. Rhynchotrema 

 Gapax (Richmond formation of the Upper Ordovician), for ex- 

 ample, was short-lived but of wide distribution ; it is thus a 

 good index fossil (see page 22), while LepicBna rhomboidalis, 

 persisting from mid-Ordovician to Mississippian, Atrypa retic- 

 ularis, throughout the Silurian and Devonian, and Produdus 

 semireticulatus, ihToughout the Mississippian and Pennsylvanian, 

 were the brachiopod Methuselahs of the Paleozoic ; these forms 

 are accordingly almost valueless as indicators of the age of their 

 inclosing strata. 



Brachiopods are known since the earliest Cambrian, and 

 therefore the class must have originated in the pre-Cambrian ; 

 the class reaches its maximum of differentiation in the Silurian 

 and Devonian, with the dying out of stocks in the late Paleo- 

 zoic. Since Mesozoic times the rhynchonellids and terebratu- 

 lids have dominated, and along with them a few lingulids, dis- 

 cinids, cranids and strophomenids have persisted. The class 

 is tending slowly toward extinction. 



Derivation of name : Brachiopoda > Greek brachion, arm, + 

 pous (pod), foot. The arms (brachia) were formerly thought 

 to be organs of locomotion. 



The Brachiopoda are subdivided into two sub-classes : 



1. Inarticulata. 



2. Articulata. 



Sub-Class i, Inarticulata 



Valves held in apposition merely by muscles, there being 

 usually no teeth or sockets to hold the two \^alves firmly together 



