2 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



we are likely to be led astray rather than aided by any detailed comparison with fossil 

 types. 



However, I believe it only fair to my colleagues to give a summary of their work 

 and of their views, without further comment, so that future workers may be able 

 to judge for themselves the relative merit of the various opinions expressed. 



PINNULES AND ARMS 



Dr. Gisle'n, in common with Dr. Th. Mortensen, disagrees with my interpretation 

 of the pinnules of the recent crinoids. Mortensen said that, guided by his researches 

 and observations on pathological and embryological material, he considered himself 

 bound to uphold the theory of P. H. Carpenter that pinnules correspond morpho- 

 logically to dwarfed arms. 



Gisle'n noted that Bather had stated that the only difference between pinnules 

 and arms is that pinnules contain fertile gonads. But he pointed out that this dis- 

 tinction does not always exist, for many stalked crinoids and even comatulids, as 

 for instance Notocrinus and Comatula, have fertile gonads in the arms. Furthermore, 

 distal pinnules, and all the pinnules of the anterior radii of many comasterids, also 

 lack fertile gonads. He said that the only difference that should be made use of in 

 a definition of the different types of brachial ramification is that the pinnule is a small 

 unramified arm, as a rule considerably shorter than the main arm. He noted that, 

 so far as he had observed, the articulations between the pinnule segments in recent 

 crinoids are always unlike those between the brachials, if the articulations between 

 the first two segments are excepted. Syzygial articulations between the pinnulars 

 are found in Hyocrinus, and also between the segments of P 2 in Stephanometra. 



In Hyocrinus, as Gisle'n remarked, the pinnules are very long, approaching the 

 main arm in size, and therefore the pinnule-bearing brachials may have the appear- 

 ance of axillaries. When pinnules are of nearly the same size as the arms that bear 

 them Gisle'n said it is a matter of opinion whether they are to be regarded as pinnules 

 or as arms. 



He concluded that the phylogenetic development has doubtless been that an 

 isotomic type of ramification was succeeded by a heterotomic, which in its turn was 

 replaced by a metatomic. The lateral ramifications began to appear at regular inter- 

 vals, diminished in relative size, and became unramified; thus the holotomic or 

 pinnulate type was reached. The arm of the recent crinoids, according to Gisle'n, 

 should thus be regarded as a sympodium that has arisen in the following way: Right 

 and left ramifications alternately have remained at their full development, while 

 corresponding left and right ramifications have been suppressed into pinnules. 



Gisle'n gave a detailed account of the phylogenetic development of pinnules from 

 arm ramifications, based chiefly upon some of Angelin's type specimens in the Riks 

 Museum at Stockholm. 



Mortensen showed that Dr. W. B. Carpenter's account of a sympodial growth 

 of the arm during ontogenetic development is incorrect. The pinnule segments are 

 weaker from the very first, though on the other hand their longitudinal growth is at 

 first more rapid. 



From an examination of specimens of Comatula pectinata and of Comanthus 

 parvicirra Gisle'n found that in the anterior radii the arms terminate in the manner 



