2 Ki LLBTTN 82, I \tri.n STATUS NATIONAL Mi si; r\! 



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

 typi 



Bowever, I beheve il only fair to my colleagues to give a summary of their work 

 and of tlieir 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. Gislen, in common with Dr. Th. Mortensen, disagrees with my interpretation 

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

 and observations on pathological and embryological material, he considered himself 

 hound to uphold the theory of P. II. Carpenter — that pinnules correspond morpho- 

 Logically to dwarfed arms. 



Gislen noted thai 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 erinoids and even comatulids, as 

 for instance Notocrinil8 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 

 unramilied 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 

 erinoids 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 Gislen 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 Gislen said it is a matter of opinion whether they are to be regarded as pinnules 

 or as arms. 



lie 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 erinoids, according to Gislen, 

 should thus he 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. 



i\e 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 

 lir-t more rapid. 



From an examination of specimens of Cmnntula ptdinata and of Comanihux 

 parvicirra Gislen found that in die anterior radii the arms terminate in the manner 



