STRONGYLOCENTROTID.E. 339 



In the pedicellarice, the differentiation of special forms undergoes a series of 

 changes identical with that which exists in the Echinidse. Thus in the simplest 

 forms, those with only four pairs of pores and all oculars exsert, the globiferous 

 pedicellarise, like those found in the most primitive species of Psammechinus 

 and Echinus, have valves with lateral teeth on both sides. In forms somewhat 

 more specialized, a lateral tooth on only one side is found and in many species 

 that has disappeared and there is only the conspicuous terminal tooth, so char- 

 acteristic of the most specialized Echinidse. It is interesting to find that many 

 of the more specialized species of Strongylocentrotidse have retained the less 

 specialized form of pedicellaria? ; thus Loxechinus albus, a highly modified species, 

 has the simple form of globiferous pedicellarise. On the other hand, Strongylo- 

 centrotus granularis, which is very variable but without highly specialized am- 

 bulacra, has the most extreme form of pedicellarise. There do not seem to be 

 any characters in which the pedicellarise of the Strongylocentrotidse differ 

 regularly from those of the Echinidse. It is neither necessary nor desirable 

 therefore to repeat here their characteristic features. Usually all four kinds are 

 present on any given specimen; not infrequently the tridentate occur in two 

 forms and in a few species, there are two forms of globiferous. In some species 

 the globiferous pedicellarise are quite wanting, at least in adult specimens, but 

 they are usually common enough. 



In their sphceridia, this family shows no special advance over what is found 

 in the Echinidse. There are several to many sphseridia on the actinal portion 

 of each ambulacrum, attached to minute tubercles, not at all sunken in depres- 

 sions in the test. They vary in shape from almost globular to oval or ellipsoidal. 

 They are usually smooth but may be quite rough at the tip. 



The spicules in the tube-feet and in the glands of the globiferous pedicellarise 

 are usually bihamate, with the ends simple or more or less branched; often the 

 spicules are not developed completely but are short, slightly curved with rounded 

 ends. What Mortensen calls "biacerate" spicules also occur. As any two or 

 three of these forms may occur in a single specimen, they are without any real 

 systematic importance. 



THE GENERA AND SPECIES OF STRONGYLOCENTROTID.E. 



In attempting to arrange the species of this family in anything like a natural 

 sequence, the genera, with the exception of Echinostrephus and Pseudoboletia, 

 which are really a little off the main line of development, are exceedingly hard 



