90 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [bulls?. 



are first introduced during adolescent growth or senility, and these 

 by the law of acceleration appear earlier and earlier in later spe 

 cies. In the Lower Cambrian there are species of Billingsella witli a 

 few broad undulations in the shell, but in the Middle Cambrian tlie 

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

 portion of the valves, while in the Upper Cambrian these folds ai)pear 

 upon the umbones. In the oldest rostrate pentanieroids the shells are 

 either smooth or have a few folds (Camarella), which beconn^ more dis 

 tinct in Parastrophia, and culminate in numerous sharp plications in 

 Auastrophia. The rhynchonelloids, 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 PROTEGULUM. 



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 common form of 

 embryonic shell, which may be termed the proteguhim. The protegulum is semi- 

 circular or semielliptical in outline, with a straight or arcnate hinge line, and 

 no hinge area. A slight posterior gaping is produced by the ventral valve being 

 usuallj'^ more convex than the brachial. The modifications 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 Uiscinisca. 



As the protegulum has been observed in about 40 geuera, representing nearly all 

 the leading families of the class, its general presence may be safelj' assumed. [In 

 structure it is corneous and imperforate and varies in size from 0.0.5 to 0.60mm. 

 The] prototype preserving throughout its de\elopment the main features of the 

 protegulum, and showing no separate or distinct stages of growth [is found in the 

 Lower Cambrian genus Pateriua]. The resemblance of this form to the protegulum 

 of other brachiopods is very marked and significant, as it represents a mature type 

 having only the common embryonal features of other geuera. 



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

 type species of Paterina has a well-developed cardinal area, and that it 

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

 to have an af)ical pedicle opening as in the Acrotretidiie. 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. Ijihidea is therefore in harmony with 

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

 The theoretical Paterina or prototype of the protegulum is^therelbre 



> Am. .Jour. Sci., April. 1891, 3(1 series, Vol. XLI, pp. 344-346. 

 2Proc. U. S. Nat. Mua., Vol, XIX, 1897, pp. 707-713. 



