114 SYNOPSIS OF AMERICAN FOSSIL URAOHIOPODA. [iiru-.R?. 



in 1850, Hall in 1860, and by several authors since, and has commonly been termed a 

 visceral foramen, (l^hlert snggests that it was probably occupied by IIks 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 present in Athyris, llensselM-ria, 

 Cryptonella, etc., is a remnant of this aperture, the remainder of tlie 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 alimentaiy canal, and 

 if this were true, then the deep and narrow median chamber bounded by the crural 

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



Ill 1834 You Bucb also divided the class into two sections, founded on 

 the mode of attacliraeut. 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, the 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; 

 (h) ventral vahe perforated for the protrusion of tlie pedicle; and (c) 

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

 ever, is identical with ?>, since Leptsena, 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- 

 champs 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 difterentiation. 

 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 througli the ^'oluminous 

 writings of Thomas Davidson. 



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

 Brachiopoda into two subclasses, Ancylopoda and Ilelictopoda. 

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

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

 guished in having the "oral arms recurved and aftixed to fixed 

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

 topoda "they are regularly spirally 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 loo]vbearing genera, 

 as Cistella and Gwynia. Therefore Ilelictopoda, as far as the brachial 

 structure is concerned, will also include the Ancylopoda. In fact, 

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



'Pal.-pontdlouy of New York, A'ol. VIIT, Part II, 1895, p. 334. 



