NO. 7 SEA-LILIES AND FEATHER-STARS — CLARK 1 3 



which to a large degree hmits the extension of the pentacrinites and 

 such comatuhds as the Thalassometrinse and the Charitometridffi 

 toward the surface, since as a result of wave action breakage is most 

 common in the littoral and without a large chambered organ a crinoid, 

 unless unusually tough, could not repair its injuries with sufficient 

 rapidity to survive. 



The most common mutilations are the loss of the visceral mass and 

 of more or less important portions of the arms, the fracture in the 

 latter case almost invariably occurring at a syzygy. Loss of the 

 visceral mass appears to cause the animal no inconvenience whatever, 

 and it is entirely replaced in a little more than three weeks. 



ASYMMETRY 

 There are four types of deviation from the normal pentamerous 

 symmetry of the crinoids. These follow the following lines : 



I. A rearrangement of the five primary groove trunks upon the 

 disc whereby (a) the left posterior increases in size and gives ofif 

 more branches than any of the others ; (b) as a result of the anterior 

 migration of the mouth the two posterior become much longer and 

 the anterior much shorter than the others and a condition of bilateral 

 symmetry is attained; (c) correlated with the anterior migration of 

 the mouth, all of the primary groove trunks become merged into a 

 horseshoe-shaped ring which skirts the lateral and anterior borders 

 of the disc, giving off branches to the arms, the mouth being in the 

 right center of the ring so that the ambulacra on the left are more 

 developed than those on the right, or the ambulacra leading to the left 

 posterior ray disappear altogether so that the ambulacra on the right 

 are more developed than those on the left. 



. 2. A dwarfing, or an overdevelopment, of the left posterior, more 

 rarely of both posterior, radial with the accompanying post-radial 

 series. 



3. The intercalation of additional radials and post-radial series 

 which alternate with the original five, and the associated dropping out 

 of one of the five radials. 



4. The suppression of two of the primarily five basals. 

 Asymmetry is almost universal in the comatulid family Comas- 



teridse, which includes the most specialized of all the recent types ; in 

 this family the first and second types occur, though the latter is much 

 less common. 



Asymmetry is characteristic of the genus Promachocrimis, which 

 is probably rightly considered as the most specialized genus in the 

 subfamily Heliometrinse ; here the first and third types occur. 



