10 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



In 10 additional specimens in the Copenhagen Museum the first pinnule is on the 

 right of the second brachial in 50 cases, and on the left in 48. Gisl6n said that it 

 might be supposed this peculiarity might be accounted for by the genus being 10- 

 rayed ; but the same distribution may also be seen in the 5-rayed young. In Thau- 

 matocrinus renovatus PI appears to the right of the second brachial in three cases and 

 to the left in two. The position of P! also varies in a young individual of T. jungerseni. 



In Pentametrocrinus and Atopocrinus the first syzygy is usually between brachials 

 4 + 5, and the first pinnule is on either the second' or the fifth brachials. 



Carpenter said that in Pentametrocrinus japonicus the lowest pinnule (on the 

 fifth brachial) appeared to the right in 11 cases out of 12. P! also appeared to the 

 right on the second brachial in P. varians. It is also stated to be commoner on the 

 right than on the left in P. semperi. Gislen examined two specimens of P. diomedeae; 

 in these the lowest pinnule was on the right in 9 cases out of 10. Judged from Koehl- 

 er's figures the same seems to be the case in P. atlanticus. In Atopocrinus sibogae P, 

 in 4 cases out of 5 is to the right on the second brachial. 



Gislen gave some examples of the suppression of a right or a left arm in other 

 comatulids. The examples given showed a tendency for the suppression to be located 

 differently in certain oligophreate families on the one hand and certain rnacrophreate 

 types on the other. 



In a 9-armed specimen of Comatula pectinata from Java the left arm, as in Eudio- 

 crinus, is rudimentary. It is represented by a small calcareous lump of two ossicles 

 united by syzygy. The right arm has the first 6 brachials united hi 3 syzygial pairs. 



Springer described a 9-armed specimen of Uintacrinus socialis in which a left arm 

 is suppressed into a pinnule. 



Gislen found a similar case in a 9-armed specimen of Antedon petasus. On the 

 undivided arm the second and third ossicles are united by synarthry, and the fourth 

 and fifth and tenth and eleventh are united by syzygy. The undivided arm is as stout 

 as the others. On both sides of the arm the first gonad is on the third pinnule. The 

 unusual distribution of nonmuscular articulations is the same as in the specimen of 

 Uintacrinus socialis just mentioned. According to Gisl6n these cases may be ex- 

 plained in two ways either the reduction has gone so far that both the suppressed 

 arm and the ossicle to which it was attached have disappeared, or the arm alone has 

 been suppressed, the IBri and IBr 2 having been coalesced into a single ossicle. Since 

 there is a muscular articulation between the first and second ossicles following the 

 radial, the second alternative is believed by Gisle"n to be the more probable. In one 

 of these cases it is clearly a right arm and in the other a left arm that has been sup- 

 pressed. Gisle'n said it is to be desired that more data regarding the pinnulation of 

 similar forms with undivided postradial series should be made available in order to 

 enable us to decide whether the tendency to suppression is more frequent in one or 

 the other, or, as in Thaumatocrinus, equally frequent in both. 



Gislen noted that during ontogenetic development the first arm ramification is 

 formed by two equally stout arms. Sometimes, however, it seems that a difference 

 in length between the two arms which is very insignificant, and in the larger young 

 soon disappears, arises, at least in certain cases. Thus we have here a difference in 

 length between the two arms of a pah", not a difference between pairs of arms such as 

 is illustrated and described by Perrier. This last, indeed, seems not to occur. The 



