A MONOGRAPH OF THE EXISTING CRINOIDS 9 



The difference is that Pi and the first arm ramification developed to the right in 

 Metacrinus, while they are found on the left in Calamocrinus. 



The other genera of the family Hyocrinidae are, like Calamocrinus, defective in 

 their pinnulation. Information regarding the position of the pinnules is sporadic and 

 incomplete. A reconstruction of the defective rows of pinnules shows, however, that 

 in the great majority of cases the suppressed P x must have been on the left of the 

 second brachial. A specimen of Ptilocrinus pinnatus examined had, up to the eleventh 

 brachial, on all five arms syzygies between brachials 1+2, 5 + 6, and 8+9, with pin- 

 nules on brachials 4 (left), 6, 7, 9, 10, and 11. Of Gephyrocrinus there are only three 

 specimens known. The pinnulation and distribution of the syzygies in the arm bases 

 up to and including the sixth brachial are for the most part as in Ptilocrinus. The 

 first pinnule is on the fourth brachial. According to the description and figures the 

 first (suppressed) pinnule was on the left of the second brachial in 10 cases out of 11. 

 In Thalassocrinus the first developed pinnule is on the fifth brachial, and on the left. 

 In Hyocrinus bethellianus the first pinnule is on the left of the sixth brachial. Accord- 

 ing to Gislen Carpenter's figure of this species is reversed. 



In the Phrynocrinidae the genus Naumachocrinus is known only from a single very 

 defective specimen. In Phryriocrinus the first arm ramification is at about the 

 twentieth-twenty-fifth brachial. The lowest pinnule is on the seventh or eigth 

 brachial, to the right in two cases and to the left in three. A reconstruction of the 

 pinnulation on the five rays of the only known specimen of Phrynocrinus nudus shows 

 that the (suppressed) Pi was on the right of the first epizygal in three cases, and on 

 the left in two. 



The family Bathycrinidae is represented in the recent seas by six genera, of which 

 three have 10 arms with IBr series of two elements, and three have five undivided 

 arms — Rhizocrinus, Bythocrinus, and Democrinus. 



The genera of Bathycrinidae fall into two series, one with short and coalesced 

 basals, the other with very long basals joined by suture. Gislen remarked that it is 

 interesting to note that the position of the first pinnule is different in these two series. 



In Rhizocrinus lofotensis the first developed pinnule appears almost always on 

 the eighth brachial, and usually to the right — in 22 cases out of 30 in specimens from 

 the Trondhjem Fjord. Pi would therefore, if developed, usually appear on the left 

 of the second brachial. 



In the genus Bythocrinus, on the other hand, the arrangement is different. In 

 Bythocrinus cf . braueri in the four specimens studied the first pinnule in 9 cases out of 

 11 appears to the left on the fourth brachial, or to the right on the sixth brachial. 

 Therefore P] would, if developed, usually appear on the right of the second brachial. 



In the comatulids four genera have undivided arms. In the only one of these 

 four genera belonging to the oligophreate type, Eudiocrinus, the first pinnule is 

 invariably on the left, whereas of the three remaining genera, belonging to the Macro- 

 phreata, at least two have the first pinnule to the right in the great majority of cases. 



The three macrophreate genera with undivided arms are Thaumatocrinus, with 

 10 arms, and Pentametrocrinus and Atopocrinus, each with 5 arms. 



In Thaumatocrinus Gislen found that the first pinnule is as often to the right as 

 to the left in the species he was able to study. In the specimen of T. jungerseni in 

 the Upsala Museum P x is on the right of the second brachial in five cases out of nine. 



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