I032 PRINCIPLES OF STRATIGRAPHY 



Pentacrinus asterius L., 80 fathoms (range 80-320 fathoms 



equals 146-584 meters). 

 P. decorus Wy. Th., 84 fathoms (range 84-667 fathoms equals 



153 to 1,219 meters). 

 P. mulleri Oerst, 84 fathoms (range 84-531 fathoms equals 153 



to 970 meters). 

 Promachocrinus kerguelcnsis Carp., 28 fathoms (range 28-120 



fathoms equals 51-218 meters). 

 Rhizocrinus lofotensis Sars, 80 fathoms (range 80-1,900? fath- 

 oms equals 146-3,474 meters). 



R. razvsoni Pourt., 73 fathoms (range 73-1,280 fathoms equals 



133-2,340 meters). 



The egg of Antedon develops into an egg-shaped mero-plank- 

 tonic larva, which has a tuft of long flagella on the anterior end 

 and five ciliated rings surrounding it ; no mouth or anus is present. 

 This embryo swims about for a period of time, varying from a few 

 hours to several days, and, on settling down to a benthonic life, 

 attaches itself at a point on the ventral side between the first and 

 second ciliated rings. The whole anterior part, as far as the third 

 ciliated ring, becomes the stalk, the posterior part developing into 

 the calyx. In Antedon the stem is retained only during the early 

 stages of development, the adult animal being free. 



Asteroidea; Ophiuroidea. These belong to the marine vagrant 

 benthos, living mainly in shallow water or in moderate depths, 

 though some species descend to depths of 2,000 fathoms or over. 

 Some littoral starfish can undergo an exposure for several hours 

 in regions laid bare by the tide. A muddy or sandy bottom seems 

 to be the most characteristic facies for these animals, and from 

 such bottoms hundreds are often brought up in a single haul of 

 the dredge. Their relative scarcity in beds in which they are known 

 to occur is probably due to the fact that, after the death of the ani- 

 mal, the skeleton quickly falls apart into its component plates, which 

 become separately embedded in the sediments. In the majority of 

 the Asterozoa meroplanktonic, bilaterally symmetric, ciliated larvse 

 occur, which in the Asteroidea are known as bipinnaria and brach- 

 iolaria, and in the Ophiuroidea as pluteus. These are often found 

 in great numbers in the pelagic fauna. 



EcHiNOiDEA. The Echinoids, or Sea Urchins, are without ex- 

 ception marine vagrant benthonic animals, living usually in large 

 numbers in moderate depths. A few species descend to depths 

 between 2,000 and 3,000 fathoms, but the majority prefer the 

 shallow portions of the littoral districts. On the coast of Maine 



