524 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ANTEDON BiriDA. 

 Figs. 1191-1193, pi. 30; 1195, pi. 31; 1201-1203, pi. 32; and 1204-1206, pi. 33. 



The pentacrinoids of this species in their later stages have already been 

 described, and the earlier stages are considered in the section dealing with 

 embryology and development (p. 502). It will be sufficient here to recall the 

 following facts: 



The total length of the fully grown pentacrinoids from the base of the column 

 to the extremities of the arms when these are folded together is 0,7 inch; the 

 column alone measures 0.25 inch ; when the arms are fully expanded the diameter 

 of their circle is about 0.5 inch. 



There appear to be no infrabasals. 



The columnals number 16-27, averaging about 20. 



Prof. Michael Sars, who described the pentacrinoid of Hathronietra sarsii, 

 was never able to secure pentacrinoids of Antedon petasus with which to compare 

 them, but through friends he obtained 17 specimens of the pentacrinoids of Antedon 

 bifida from Belfast, Ireland, and from the Firth of Clyde, of which he gave a 

 description for purposes of comparison with the pentacrinoids of II. sarsii. 



The most developed of these pentacrinoids of Antedon bifida was 10 mm. 

 long; the column, composed of 16 segments, measured 4.25 mm. In three others 

 of approximately the same size the stem was composed of 16, 21, and 27 segments 

 In one of these three, of which the length was also 10 mm., and in which the column 

 was relatively longer than in the others, being nearly 8 mm. long and consisting 

 of 27 segments, the crown, which scarcely exceeded 2 mm. in length, was only 

 slightly developed, the arms being without pinnules and the centrodorsal without 

 cirri. 



The specimens in which the arms are the most developed and possess two or 

 three pairs of pinnules near the distal extremity have the centrodorsal with 5, 7, 

 or 9 cirri of different sizes; in one it bears a little tubercle, indicating the 

 beginnings of a tenth cirrus. 



The five primary cirri are, as stated by W. B. Carpenter, situated interradially. 



P t begins to develop when the rest of the arm has only two or three pairs 

 of pinnules near its distal end. 



In one of the largest individuals the periphery of the terminal stem plate 

 shows some very short digitiform prolongations as in the pentacrinoid of Hathro- 



metra sarsii. 



While in the case of Antedon bifida very often several pentacrinoids may 

 grow so closely together that they appear to arise from a common base, this never 

 happens in H. sarsii. 



Professor Sars thus summarizes the difference between the pentacrinoids of 

 Antedon bifida and those of Hathrometra sarsii. 



Though when adult Hathrometra sarsii is no larger than Antedon bifida its 

 pentacrinoid young is very much larger, being from three to four times as long. 

 The column is relatively much longer, but the arms are shorter. 



