8 BULLS UN 82, I N I I ED STATES NATIONAL MUSEUM 



thai i" the righl bearing externally a normal undivided arm and internally a IIIBr 11 

 (3 + 4 1 Belies, on which the outer arm lias the proximal syzygies between brachials 4+5, 

 g • in. L3 : ll. and 17 • 18, and the inner lias them between brachials 2+3, 7+8, and 

 11+12. 



(Helen said that in the firsl two cases it is an inner pinnule thai lias developed 



into an arm. The Brsl example is perhaps the must remarkable, presenting a picture 

 of the repetition, bo far as possible normal, in more distant parts of the arm of the 

 proximal arm ramification and its distribution of aonmuscular articulations. 



Gisleh ooted thai Mortensen (1920) has described a ease of P 3 developing into an 

 arm m Antedon petasus, and that the author has described and figured an arm of 

 Thaumat jp. forked twice in its distal portion. The proximal fork, he said, 



seems to be due to a mere splitting of the growing point, but the distal branch seems 

 to have 1 a caused by an hypertrophied pinnule. 



Gisleh said we have already seen how the new arms in the genus Metacrinus 

 develop in normal cases on the inner side of a main arm. and asks: What, then, is 

 the condition at the firsl ramification, the I Br axillary? Here there can be no question 

 of the new arm developing on the inner side of an arm branch. A comparison of the 

 material thai has been considered shows that in a great majority of cases it is the right 

 hand pinnule thai has been strengthened and has become equivalent to the main 

 arm. corresponding to a IBr 7(1+2, 4+5) series in M. ink rruptus. In 9 specimens of 

 \f. nobilis tenuis the right pinnule is strengthened into an arm in 24 cases, the left 

 pinnule in 11. In 7 specimens of M. r itmulus 25 right and 1 1 left pinnules are strength- 

 ened into arms. In 19 specimens of M. inti rrujitus 73 right and 15 left pinnules are 

 developed into arms. In all 122 right pinnules as against 37 left pinnules have been 

 strengthened. Gislen said that the reason for this may be ascertained. In by far 

 t he greater number of eases the firsl pinnule is on the right side of the second brachial — 

 in other words it is the right branch that is suppressed at the first ramification. When 

 the first real arm ramification at last takes place, it is on the right side that the sup- 

 pressed impulse to arm formation is stronger, and therefore it is oftener a right than a 

 left pinnule that develops into an arm. In 9 specimens of M. nobilis tenuis Pi is 

 nil the right side of the second brachial in 24 cases out of 35, in 7 specimens of M. 

 rotundus in 26 cases out of 30, and in 21 specimens of M. interruptus in 85 out of 100 — 

 in all, in 135 cases out of 165. Thus it is the right arm that is suppressed in the genus 

 Metacrinus. 



In the genera of the family Hyocrinidae, according to Gislen, Calamocrinus has its 

 first arm ramification after the first pinnule, while the other genera have simple arms. 

 Examples of the arm base in Calamocrinus up to and including the first axillary are: 

 IBr KKl+2, 5 + 6, 7 + 8), with pinnules on brachials 4 (left), 6, 8, 9; and IBr 10 

 1 2, 5 + 6, 8 + 9), with pinnules on brachials 4 (left), 6, 7, 9. The first pair of pin- 

 nules is thus completely suppressed, and the first pinnule develops in the great 

 majority of cases to the left of the fourth brachial the suppressed first pinnule would 

 also have appeared on the left (of the second brachial) if it had developed. The first 

 arm ramification is also usually caused by the strengthening of a left pinnule. Thus 



here also there is a connection between the development of the first pinnule and of the 

 lirst arm ramification similar to that which may be made out in the genus Metacrinus. 



