166 DE. T. DAVIDSON ON EECENT BEACHIOPODA. 



jirocesses ; they can bo uurolled at the will of the auimal and protruded to some distance 

 beyond the margins of the valves, contrary to the statement made by Barrett. When 

 forci])ly stretched out they are said to be more than four times the length of the shell, 

 and support some 3000 cii'ri. In a note on the extension of the coiled arms in Rhyn- 

 chonella j:>si(tacea, published in the ' Amer. Journ. Sci. & Arts,' 3rd ser. vol. xvii. 1879, 

 p. 257, Prof. Morse says : — " Years ago von Buch recorded that Otto Erederic Miiller had 

 observed Rliijnclionella j^sittacea protrude its arms beyond the anterior borders of the 

 shell. This single observation was not widely accepted, and many doubted the possibility 

 of the arms being exerted in this manner. In the year 1872, Avhile studying living 

 Rliynchonella in the St. Lawrence, I observed a specimen protrude its arms to a distance of 

 four centimeters beyond the anterior borders of the shell, a distance nearly equaling twice 

 the length of the shell. This year I again had an opportunity of studying Rhynclionella in 

 Hakodate, Yesso, and again observed the same features. Specimens lying on the bottom of 

 a glass dish protruded their arms a short distance, and remained in this position for hours. 

 .... The movements of the arms were very sluggish, though the cirri were constantly 

 in motion. Sometimes the shells closed upon the arms before they were retracted." 



In 1833 Owen [1. c.) gave a figure in illustration of the arms, one of which has been 

 artificially unfolded. Space will unfortunately not admit of my reproducing all the 

 admirable details in connection with the anatomy of R. j^s/ttacea so well described and 

 illustrated by A. Hancock ; we will, however, refer to the most important features. 



" In Rhynchonella psittacea (p. 799, I. c.) the general disposition of the muscles is the 

 same as in W. mistralis, only they are longer and more slender, particularly the occlusors, 

 Avhich, with the exception of their extremities, are thin and tendinous. The surfaces of 

 attachment also vary a little in form, as can be readily seen on removing the shell. The 

 extremities of the muscles are then observed, as usual, at the ventral surface of the 

 animal, clustered together, a little in advance of the umbonal region. The occlusors are 

 almost colourless, narrow and elongated, and have their anterior margins united on the 

 median line ; posteriorly they diverge, leaving a space between them, in Avhich the 

 terminal extremity of the intestine is seen. The divaricators are very large, and are 

 placed external to, and in contact with, the occlusors ; they are broad and fan-like in front, 

 exhibiting radiating divisions, and meet on the median line, in advance of the occlusors. 

 Behind they are produced and pointed. The ventral adjuster muscles have their 

 extremities narrow and much elongated ; they rest agaiust the external margin of the 

 divaricators, are enlarged a little, and converge posteriorly. Unlike Waldheimia, there 

 is here a pair of peduncular muscles seen at the sides of the umbo, having between 

 them the accessory divaricators, which are of a somewhat triangular form. The dorsal 

 terminations of the occlusors present nothing remarkable in their appearance. All the 

 extremities of the muscles seen at the surface of the animal are of a yellowish-red coloux*, 

 except those of the peduncular muscles and the ventral terminations of the occlusors . . . 

 The muscles (l. c. p. 805) in the Terebratididce and RhynGlionellklce are peculiar for their 

 enlarged, fleshy extremities, and for the attenuated, tendinous character of their inter- 

 mediate portions . . . 



*' The arms [p. 802] of R. psittacea are totally deprived of calcareous support except at 



