CLARK : RECENT CRINOIDS 583 



a sufficient knowledge (the Pentacrinitida and the Bourgueti- 

 crinidae) are also found at these temperatures, while three of the 

 others (the famihes Apiocrinidae, Phrynocrinidae and Plicato- 

 crinidae) are found below, and one (Holopodidae) is known only 

 from above. 



Among the comatulids we are without sufficient data in regard 

 to five famines (Himerometridae, Stephanometridae, Tropiometri- 

 dae, Ptilometrinae and Atelecrinidae) ; but one of these (Ptilome- 

 trinae) is actually known from this temperature, while the four 

 others are without doubt found here, if we may judge from the 

 mean temperature of the sea at the depths at which they are 

 known to live. At least all except four, and with little doubt 

 all, of the comatulid families are found between 55° and 65°, and 

 not onlj^ these, but also the two dominant families of the stalked 

 crinoids (Pentacrinitida and Bourgueticrinidae) . Thus it ap- 

 pears that the range of temperature between 55° and 65° (12.78° 

 and 18.33°C.) represents the temperature most suitable for the 

 recent crinoids. 



There are certain very interesting attributes of the crinoids 

 which are found between the temperature limits of 55° and 65°; 

 they are all of medium size, none being very large and none very 

 small, and they are all conservative in their characters, with 

 never a large number of arms — usually ten only. This would 

 appear to indicate that the range of temperature between 55° 

 and 65° represents the temperature physiologically most suitable 

 for the crinoids, a temperature which tends to maintain a phy- 

 logenetical conservatism and to suppress any tendency toward 

 the extreme type of development characteristic of the crinoids of 

 the warm water, as well as that characteristic of the crinoids of 

 the cold water. 



So far as can be seen, it is with recent species found between 

 these temperatures that the fossil crinoids best agree, and thus 

 the suggestion may be made that it was principally, if not en- 

 tirely, within this temperature range that the crinoids of the 

 post-palaeozoic fauna, which was characterized by a very great 

 development of the present dominant order, the Articulata, were 

 developed. 



