90 SYNOPSIS OP AMERICAN FOSSIL BRACHIOPODA. [bull. 87. 



are first introduced during adolescent growth or senility, and these 

 by the law of acceleration api)ear earlier and earlier in later spe- 

 cies. In the Lower Cambrian there are species of l^.illingsella with a 

 few broad undulations in the shell, but in the ^Middle Cambrian tiie 

 plications are pronounced and cover half or more than half the anterior 

 portion of the valves, while in the Upper Cambrian these fohls appear 

 upon the umbones. In the oldest rostrate i)entanien>ids the shells are 

 either smooth or have a few folds (Canmrella), whicli become more (lis 

 tinct in Parastrophia, and culminate in numerous sharp plications in 

 Anastrophia, The rliynchonelloids, beginning in Protorthis of the 

 Lower Cambrian as smooth shells, gradually become more and more 

 plicated in the Silurian and Devonian, yet in the Triassic many species 

 again appear nearly smooth. 



STRUCTURAL CHARACTERS. 

 THE PROTEGULUil. 



The order Atremata is the radical brachiopodous stock, which early 

 in its history gave origin more or less directly to the other three orders 

 of brachiopods. Beecher has observed : ' 



That all brachiopods, so far as studied by the writer, have a comiuoii tonu of 

 embryouic sliell, which may be termed the protegulnm. The piotenulum is semi- 

 circular or seuiielliptieal in outline, with a straight or arcuate hinge line, and 

 uo hinge area. A slight posterior gaping is produced by the ventral valve being 

 usually more convex than the brachial. The moditications noted are apparently due 

 to accelerated growth, by which characters primarily nealogic [=neanic] become so 

 advanced in the devolopment of the individual as to bo impressed finally upon the 

 embryonic shell. This feature is well shown in the development of Orbiculoidea 

 and Discinisca. 



As the protegulnm has been observed in about 40 genera, representing nearly all 

 the leading I'aniilies of the class, its general presence may be safely assumed. [In 

 structure it is corneous and imperforate and varies in size from 0.05 to 0.60mni. 

 The] prototype preserving throughout its deveb>pmeut the main features of the 

 protegnluni, and showing no separate or distinct stag^js of growth [is found in the 

 Lower Cambrian genus Faterina]. The resemblance of this form to the ])r(>tegnlum 

 of other l)racliiopod8 is very marked and signilieant, as it represents a mature tyjie 

 having only the eonunon embryonal features of other genera. 



Since the above was written Mr. C. D. Walcott has shown tliat the 

 type species of Paterina has a well-develoi)ed cardinal area, and that it 

 is synonymous with Iphidea.^ The latter, however, is generally assumed 

 to have an apical pedicle opening as in the Acrotretidie. This is now- 

 known not to be the case. The supposed perforation is but a slight 

 depression or short groove in the apex of the ventral valve, and does 

 not pass through the shell. Iphidea is therefore in harmony with 

 Pateriim, since both have more or less well-developed cardinal areas. 

 The theoretical Paterina or prototype of the protegulnm is therelbre 



' Am.Jonr. Sci., April. 1891, M series, Vol. XLI, !))>. :U4-:U6. 

 sProc. U. S. Nat. Mus., Vol, XIX, 1897, pp. 707-713. 



