266 



Th. Mortensen. 



were narrower". Judging from the material at my disposal there 

 are, however, some other, apparently more important differences in 

 the arms of the two species. The ambulacral furrow^ is in palœo- 

 crystalliis of uniform width from the mouth to the outer part of the 

 arm or, at most, there is a slight narrowing at the inner end (PI. XIV. 

 Fig. 9), while in P. typicus it is distinctly narrowed in the inner part, 

 reaching its greatest width somewhat outside the disk (PI. XIV. Fig. 7).^ 

 Further the number of tubefeet seems to be somewhat smaller in 

 palœocrystalliis than in P. typicus, as seen by the following numbers 

 and measurements. 



P. palœocrystallus 



P. typicus 



Length of R. 



7 mm 



10 — 



11 — 



Number of pairs 

 of tubefeet 



20—22 

 30 

 31 



Length of R. 



6 mm 

 11-5- 

 12 



Number of pairs 

 of tubefeet 



ca. 30 

 35 

 36 



This difference is certainly not very conspicuous, but judging 

 from the material at my disposal there seems to be a distinct differ- 

 ence herein between the two species. 



The spines in general give the impression of being somewhat 

 coarser in palœocrystallus than in typicus, which is, however, only 

 due to the preservation of the skin in a more or less contracted con- 

 dition. In the structure of the spines I do not find any reliable 

 difference, either in the adambulacral or the dorsal spines. The 

 adambulacral spines (PI. XIV. Fig. 1) I do not find so slender as 

 figured by Duncan and S laden (Op. cit. PI. II. Fig. 24). They may 

 be more or less thorny in both species. 



The pedicellariæ afford a rather distinct difference in the two 

 species, as correctly pointed out by Duncan and SI a den. When 

 Danielssen and Kor en (Op. cit. p. 37) state that "most of the 

 pedicellariæ, especially those found in the lateral surfaces of the rays 

 of Sars' original specimen, as \ve\l as, upon the rays of the other 

 specimens at our disposal, and which we have had under examination, 

 are formed, exactly, like those described and illustrated by Dr. S la- 

 den", this must certainly be due to their not having examined them 

 very thoroughly, or to the confounding of the two types. At least 

 the material at my disposal shows a not inconsiderable difference 

 in the structure of the pedicellariæ of the two species (PI. XV. Figs. 1, 



' In some specimens of P. typicus from the Kola-Fjord sent to me for examination 

 by Dr. K. Derjugin, St. Petersboui'g, after this had been sent to press, I find the 

 constriction of the ambulacral furrow towards the mouth less distinct. This makes 

 meless sure of the specific difference of the two forms. A much larger material 

 than that at my disposal will be necessary for settling this question definitely. 



