114 SYNOPSIS OF AMERICAN FOSSIL nRACIlIOT'ODA. [uull S7. 



in 1850, Hall in 18G0, audby sevoral authors since, ami has coinnionly hccn tornicd a 

 visceral foramen. (Ehlert suggests that it Avas probably occupied by the terminal 

 portion of the intestine. The persistence of the foramen .seems to indicate an anal 

 opening. 



Hall and Clarke state:' 



It has become evident, from a study of the hinge plate, that the so-called visceral 

 foramen -which perforates it, and which is often ]>rescnt in Athyris, Renssehcria, 

 Cryptonella, etc., is a remnant of this aperture, the remainder of tlio median open- 

 ing having become filled by a testaceous secretion. There is every reason to believe 

 that the visceral foramen was actually traversed by the lower alimentary canal, and 

 if this were true, then the deep and narrow median chamy)er bonmled by the crnral 

 plates mnst also have inclosed the terminal portion of the intestine. 



lu 1834 You Buch also divided the class iuto two sections, founded on 

 the mode of attacliraent. The first section contained all brachiopods 

 fixed by a pedicle to foreign bodies, while the second was restricted to 

 those forms in which there is no pedicle at maturity, tlie entire lower 

 or ventral valve being cemented to other objects, as in Crania. The 

 first section was again divided into three groups, on the basis of the 

 pedicle: (a) Pedicle emerging from between the valves, as in Lingula; 

 (b) ventral valve ])erforated for the protrusion of the pedicle; and (c) 

 uncemented shells without a pedicle opening. The third group, how- 

 ever, is identical with h, since Leiita^na, Productus, and Strophomena, 

 genera referred to section c, do possess a pedicle opening. While this 

 classification lacks a complete understanding of the features in question, 

 it is remarkable that Von Buch nearly sixty years ago, and Deslong- 

 charaps twenty-eight years later, recognized some of the principles upon 

 which the classification of the Brachiopoda is now established, viz, the 

 nature of the pedicle opening. 



Up to 184G the general external features of brachiopods served the 

 majority of authors as the essential basis for generic differentiation. 

 In that year, however, King pointed out that more fundamental and 

 constant characters exist in the interior of the shell, a fact which soon 

 came to be generally recognized, mainly through the voluminous 

 writings of Thomas Davidson. 



In 1848 Gray, probably stimulated by Kijig's paper, divided the 

 P>rachiopoda into two subclasses, Ancylopoda and llelictopoda. 

 These divisions rest entirely on the basis of the structure and the pres- 

 ence or absence of calcareous supports. The Ancylopoda are distin- 

 gui.shed in having the ''oral arras recurved and aflixed to fixed 

 appendages on the disk of the ventral [dorsal] valve,'' while in llelic- 

 topoda "they are regularly si)irally twisted when at rest.'" The 

 brachia, however, in all recent species, are recurved and more or less 

 spirally enrolled, except in some gerontic forms of looj) bearing genera, 

 as Ci.stella and (Jwynia. Therefore Helictopoda, as far as the brachial 

 structure is concerned, will also iiuilude the Ancyloj^oda. In fact, 

 to the former Gray referred only the terebratuloids, if Thecidia is 



' Palicontolosy of New York, Vol. V 1 1 r, Part II, 1895, p. 334. 



