OHAPTEPv IT I. 



bioijOGic bevelopmeis^t of the brachiopoda. 



ORDINAL DEVELOPMENT. 

 ATBEMATA. 



This order, wliicb begau in the Lower Cambrian, is represented by 

 199 species, or over W per cent of American Paleozoic brachiopods. 

 Its greatest representation, both in species and genera, was during the 

 Cambrian and Ordovician eras. A very marked decline set in during 

 the Silurian and Devonian, with almost extini-tion in the Carbonifer- 

 ous, where only Lingula and its subgenus Glossiua occur. 



The terminal families Trimerellid;e and Lingulida^ contain species 

 which attain the greatest individual growth. Lingulidu' has the long- 

 est phylogenetic history. It is the last important and most specialized 

 family of the Atremata, and manifests the greatest persistency and 

 specific differentiation. Lingula, the essential genus of the famil}', lived 

 at least from the Ordovician system through all succeeding time, and 

 is represented in modern seas. During this enormous period the only 

 change observable is that in the ancient forms the viscera occupied a 

 little more and the brachia somewhat less si^ace. 



In the more primitive types of Atremata, Obolacea, the shell is usu- 

 ally much thicker and less chitinous than in the higher or derived 

 families, Lingulacca. The shell is thickest in the Trimerellida^. and 

 thinnest in the Liiigulidie. From their mode of occurrence in rocks it 

 seems probable that Paterinida', Obolida', and Trimerelli(be ( = Obola- 

 cea) never lived in the mud or sand of the sea bottom, as did LingulidtB, 

 Lingulasmatida', and X)robably Lingulellidu' ( = Lingulacea).' The obol- 

 oids in all probability had short pedicles, while the linguloids have 

 very long i)edicles. The long, flexible, tubular pedicle of Lingula, 

 associated with the buried habit of the animal, ap])arent]y explains 



'Since all the species of Obolacea are known only as fossils, it may seem Lazardous to ascribe to 

 tlieni a mode of living dilTorout from that of Lingula. Tlioso shells liad short peduncles, are round 

 or oval, somctiiiics very gibbous, always coiuparativtly thick slioUi'd, anil not dLciJedly pliosphatic. 

 The writer has never observed any species of this su]>erfaiiiily in situ transverse to sedimentation, or 

 in other words "on edge." In the Tiingulacea the peduncle is very long, and the shells are elongate 

 quadrangular, triangular, spatulate, or acuminate, and, as a rule, are decidedly thin and phosphatic. 

 Recent Liugulas all live partially buried in the sea bottom, and not infreiiuently fossil species are 

 found in situ, on edge, with their apices downward. Lingulops and Lingula.sma also have been 

 observed situated on edge. The round, thick shells of Obolacea are strongly contrasted with the 

 elongate thin shells of Lingulacea. These peculiarities are in all probability due to mechanical 

 causes. The Linguloids, with their long, i)owerful, and flexible ])eduneles, are buried in the sedi- 

 ments, -while the posteriorly pointed shell is an adaptation to the same end, caused by the frequent 

 jicduncular pulling on that part of the valves. 

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