BRACHIOPODA. 49 



S. disjundus. These observations have not yet been verified, but it would be 

 reasonable to expect such phenomena even among Spirifers not belonging to 

 the group of Ostiolati. Yet here, as in so many other generic groups of the 

 brachiopods, it is the extreme development of a given peculiarity which serves 

 as a basis of generic distinction from forms possessing the same character in a 

 state of incipiency. 



What may have been the function of this organ in the physiology of the 

 animal is still a subject for speculation. King suggested that it might have 

 been a base of attachment for the pedicle-muscles. The pedicle, however, was 

 probably atrophied in the mature condition of these shells ; at least all means 

 of egress were obstructed, except beneath the deltidium. There is no reason 

 from analogy for assuming that the pedicle ever passed through this aperture 

 but in case it was thus extruded, Dr. King's supposition seems a plausible one. 

 If, however, the pedicle was atrophied from the closure of its normal channel 

 nearer the beak, this calcareous tube may have been an exudation encysting 

 this functionless organ. In one interesting species from the earliest of the 

 Carboniferous faunas, S. Herricki, Schuchert, there is a solid process in place 

 of a tube beneath the transverse plate, which is extended to the bottom 

 of the valve, thus forming a septum supporting the transverse plate, and 

 exhibiting in a striking manner an inclination toward the internal structure of 

 Spiriferina. 



The divergent views of King and Carpenter in regard to the punctation of 

 the shell in S. cmpidatus are well known, and the discussions may be found 

 principally in the Annals and Magazine of Natural History, and the Geological 

 Magazine for the years 1867 and 1868.* 



The late F B. Meek was the first to demonstratef that the shell substance 

 in S. cuspidatus is punctate, and probably all the species possessed of a transverse 

 plate and split tube have this shell structure. This punctation has been de- 

 scribed as " patchy ; " it is better developed or better retained in some parts of 



* The student may also be referred to Dr. Carpenter's earlier observations in his report to the British 



Association, 1844, " On the Microscojiic Structure of Shells," and to his treatise in Davidson's Introduction, 

 " On the Intimate Structure of the Shells of the Brachiopoda," 1852. 



t Proc. Academy of Ntitural Sciences, Phila., vol. ix, second ser., p. 275. 1865. 



