G HAP TEE III. 

 BlOXiOGIC DEI ELOPMEIS^T OF THE BRACHIOPODA. 



ORDINAL DEVELOPMENT. 

 ATREMATA. 



This order, which begau iu the Lower Cambrian, is represeuted by 

 199 species, or over 10 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 extinction in the Carbonifer- 

 ous, where only Lingula and its subgenus Glossina occur. 



The terminal families Trimerellidie and Linguhdic contain species 

 which attain the greatest individual growth. Lingulidie 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 family, 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 iu the ancient forms the viscera occujjied a 

 little more and the brachia somewhat less space. 



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, Liugulacea. The shell is thickest in the Trimerellidaj and 

 thinnest in the Lingulida'. From their mode of occurrence in rocks it 

 seems probable that Paterinidte, Obolida', and Trimerellida- (=Obola- 

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

 Lingulasmatid*, and probably Lingulellidie ( = Lingulacea).' The obol- 

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

 very long pedicles. The long, flexible, tubular pedicle of Lingula, 

 associated with the buried habit of the animal, api)arently explains 



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

 them a mode of living different from that of Lingula. These shells had short peduncles, are round 

 or oval, sometimes very gibbous, alwajs comparatively thick shelled, and not decidedly phosphatic. 

 The writer has never observed any species of this superfamily in situ transverse to sedimentation, or 

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

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

 Kecent Lingulas all live partially buried in the sea bottom, and not infrequently fossil species are 

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

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

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

 causes. The Linguloids, with their long, powerful, and flexible peduncles, are buried in the sedi- 

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

 peduncular pulling on that part of the valves. 



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