154 HYATT, 



the mouth of the shell being but an indefinitely widened 

 cosnoecial orifice. The innermost membrane of the man- 

 tle surrounds the digestive organs of the Terebratula form- 

 ing large blood sinuses on the O3sophagus very much as it- 

 does in the cavity of Pectinatella, except that in the last 

 no blood sinuses are defined. 



The lophophore of Terebratula bends upwards at the 

 base of the tentacles into a muscular ridge which incloses 

 the mouth passing round it on the posterior side. This 

 serves to convey the threads of food to the mouth, and 

 would be the homologue of the epistome if it were 

 properly situated. 



The epistome of Plumatella is immediately over the 

 ganglia and between the anus and mouth, whereas this 

 fold, which so nearly resembles it in other respects, is on 

 the opposite side of the mouth, and cannot, therefore, be 

 its equivalent. It may be the homologue of the calyx, 

 which has the same position with reference to the tenta- 

 cles, and is, in like manner, a fold of the lophophore. 

 Where there is such divergence in structure, however, 

 the only conclusive facts are to be found in embryology, 

 and until the solution of this difficulty is sought by the 

 study of development, it will be difficult to determine, 

 whether an organ corresponding to the calyx exists or not 

 among the Brachiopoda. 



The muscles of the Terebratulse are in three sets, the 

 occlusors, the divaricators, and the adjusters.* These 

 sets are placed with 'reference to the alimentary canal 

 and to each other as follows : the occlusors next to the 

 canal, the divaricators intermediate, and the adjusters 

 outside. These are their normal positions within the 

 visceral cavity as .determined by their bases of attach- 

 ment. The ventral (dorsal) bases of the lower pair of 

 adjusters are inside of the ventral bases of the occlusors ; 

 but this is not their normal condition, since the dorsal or 

 corresponding pair of adjusters on the dorsal (ventral) 

 side, the pedicle ends in all the allied genera, and the 

 whole length of the ventral pair in Waldheimia australis 



* Hancock, Op. cit. 



