24 PALEONTOLOGY 



This is the most primitive order, especially character- 

 istic of the Older Palaeozoic era, only a single family, the 

 Lingulida, surviving through later eras down to the 

 present day. It is divided into two sub-orders : 



1. Obolacea. Rounded and lenticular in form, with 

 short pedicle and thick shell ; probably fixed to floating 

 seaweed. These are confined to the Older Palaeozoic 

 era, and range from the most primitive of all brachio- 

 pods, Rustella of the Cambrian, which may be near the 

 ancestral form of all four orders, through the Obolidce to 

 the highly specialized Trimerettida of the Ordovician 

 and Silurian. In these last, the muscle attachments are 

 carried on platforms separated from the inner surface of 

 the valves by long and narrow cavities : these give the 

 internal casts a very complex form. In Obolus, each 

 valve has a well-marked cardinal area, that of the ventral 

 valve with a longitudinal pedicle-groove, and its umbo 

 being more prominent. 



2. Lingulacea. Elongated in form, burrowing, with 

 thin shell and long pedicle. 



These begin in the Cambrian with Lingtilella, which 

 retains the obolid characters of a cardinal area, with 

 pedicle-groove in the dorsal valve only, and of a more 

 prominent ventral umbo. It is followed in the Ordovician 

 by Lingula, in which the valves are almost equal, and the 

 pedicle groove is wanting. This genus survives to the 

 present time, and so is one of the longest-lived of all 

 genera, not only of Brachiopoda, but of all but the 

 lowliest phylum of animals. 



ORDER II.: NEOTREMATA. 



Inarticulate Brachiopoda, in which the valves acquire 

 a more conical form, and the pedicle becomes surrounded 

 by the ventral valve and comes to emerge close to the 



