NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 105 



Saccocoma. — Saccocoma (PI. 10, figs. 1-3) is chiefly interesting 

 because of its high degree of specialization. This form has well been 

 described by Jaekel (1893). In this form the centrale is minute and 

 may consist either of a new element to which' the basals have been 

 fused, or it may consist simply of the fused basals. The minute size 

 of this plate compared with the size of the cup is surprising, inasmuch 

 as in Marsupites, the other form the cup of which consists of but few 

 plates, the centrale has been so considerably enlarged. The arms of 

 Saccocoma are of great interest. Reference to the figure will show 

 most unusual modifications. The arms are comparatively long and 

 slender and in their distal portions are considerably branched. In 

 their proximal portions the most noticeable feature is the lateral 

 flange-like extensions of the arm ossicles. These are obviously 

 adapted for use in swimming. The comb-like processes of the more 

 distal ossicles probably represent modifications for a similar function. 

 This small form, as has been held by Jaekel, probabh" swam about in 

 enormous numbers in quiet lagoons. It is quite probable that as 

 opposed to Uintacrinus this genus lived in schools as do the Comatulse. 



Abnormally free types — Cardbocrinus. — Having discussed the mem- 

 bers of Type II, mention should be made of those forms which sub- 

 sequent to a complete loss of the column, evolve similar structures 

 to those had by the three genera above described. This detachment 

 from the column is unquestionably accidental in Carabocrinus , while 

 in Arachnocrinus it seems that it may be a condition that becomes 

 irregularly effective, though normal. These two genera are of very 

 considerable interest as pointing to the probable derivation of the 

 centrale. The case of Carabocrinus has already been described under 

 the discussion of Uintacrinus. This is the most perfect example of the 

 formation of a centrale where there is no possibility of the proximal col- 

 umnal taking part in its formation. The evidence showing that tliis 

 plate could not be other than an absolutely new skeletal element has 

 already been given. 



Arachnocrinus. — ^The genus Arachnocrinus offers us most remark- 

 able and unexpected structures of a similar nature. The genus is an 

 unusual dicycHc Inadunate which hved in the neighborhood of the 

 coral reefs of the Onondaga seas. Apparently a column was possessed 

 by this genus in the majority of cases, although the extraordinarily 

 heavy arms could seemingly not be held other than as resulting from 

 a special modification for an eleutherozoic habit. In the examination 

 of a number of individuals of this genus in the course of the prepara- 

 tion of a memoir on the Devonian Crinoidea of New York State, I 

 was astonished to find that no less than three individuals showed 

 absolutely no trace of column attachment. In each of these speci- 

 mens a centrale is to be observed. Arachnocrinus certainly had no 

 proximale, and it seems evident that here again no other explanation 



