NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 65 



It is among the members of Group II that we find the highest ex- 

 pression of an eleutherozoic habit among the Pelmatozoa. This 

 does not signify that among these forms we have the highest degree 

 of speciahzation and modification, although even in these respects 

 tlie crinoids are notable. We have here, however, the most perfect 

 adaptations to a free-swimming life. Such adaptations are best 

 sho^\Ti among the Crinoidea and but imperfectly developed in the 

 case of the Cystidea and Blastoidea. This is obviously due to the fact 

 that among the latter classes the brachial appendages have been but 

 indifferently developed, and are scarcely able to function as active 

 swimming organs. Among the members of this group we have a 

 wide range in the habits of the animals. The majority of the forms 

 may be classed as vagile benthos, locomotion being effected either 

 by crawling along the bottom or by more or less extensive swimming 

 movements. From such types we pass to those in which a truly 

 pelagic existence is maintained. 



Bather's classification. — Bather (1896, p. 995) considers that un- 

 stalked crinoids "fall into three distinct groups." These are: 



First, the group in which a portion of the stem remains, becoming modified into a 

 cirrus-bearing centro-dorsSl, as in^4n?«/on, J5'uf/iocri/i«s, and Thaumatocrinus. These 

 forms anchor themselves by their cirri, and though capable of crawling, climbing, 

 and swimming, do not often exercise their faculty of locomotion. Secondly, the 

 group in which either a portion of remaining stem, or the lower part of the cup (i. e., 

 basals or infrabasals), becomes solidified, usually by additional deposition of stereom, 

 into a knob, which, one may suppose, serves as ballast or as a sea-anchor; such forms 

 are Agassizocrinus, Edriocrinus, and Millericrinus pratti. Both of these groups have 

 a small calycal cavity with thick walls, and there can be little doubt but that all are 

 attached by a stem in the earlier stages of ontogeny. The third group, comprising 

 Marsupites, Saccocoma, and Uintacrinus, has no trace of a stem or of any anchoring 

 structure, but is in all respects adapted for free locomotion; the calycal cavity is 

 large in proportion to the thickness of the arms, and is enclosed by thin flexible walls. 



Classification of Group II here employed. — For the purpose of the 

 present paper I have thought it more expedient to use a somewhat 

 different classification. This grouping, like the one used by Bather, 

 is a purely physiological one — widely divergent types being placed 

 together because of a certain unity of structure in the apical portion 

 of the dorsal cup. The grouping likewise includes several Cystidea 

 and Blastoidea, which as regards their mode of acquisition of an 

 eleutherozoic habit are stinicturally comparable to the Crinoidea of 

 this division. 



Among the majority of the forms here described under Group II 

 the loss of the stem is a constant character and acquired as the result 

 of a definite evolutionary process. There are, however, several 

 crinoids where the loss of the column, though complete, appears 

 either to be a sporadic feature, or forced upon the animal by violent 

 disruption of the column. These crinoids shed considerable light on 

 94428°— Proc.N.M.vol.41— 11 5 



