208 



ing to the same group serve to emphasize the distinctness of this larval 

 type. 



This larval "family" appears to be characterized in the first stage 

 through the skeleton forming no basket-structure; the fairly elongate body 

 rod divides at the end into two rather long horizontal branches. In the 

 second stage a posterior transverse rod is formed, from which a pair of 

 short, branched posterolateral rods may issue; the latter may, however, 

 be lacking. There are four vibratile epaulets, bul no vibratile lobes. 



The rest of the Camarodonta was arranged by the present author in 

 the families Echinidae, Toxopneustidae and Echinometridae, while Clark 

 will not acknowledge the family Toxopneustidae, the forms referred to that 

 family being redistributed in the Echinidae and in a new family Strongylo- 

 centrotidae. The family of the Echinometridae is regarded by Clark as 

 containing only the polyporous, oblong forms, the other, more primitive 

 forms referred to this family by the present author being also redistributed 

 by Clark in the Echinidae and Strongylocentrotida?. According to my 

 view Clark's families Echinidae and Strongylocentrotidae are quite hetero- 

 geneous, while his family Echinometridae is quite natural, only, in my 

 opinion, not wide enough. We shall see now what the larvae will teach us 

 about this matter. 



It may first be stated that, as regards the various genera and the species 

 referred to them, Clark and I in general agree, only with regard to the 

 genus Strongylocentrotus we disagree in some important points, as is stated 

 below. 



To the family Echinidae Clark refers the genera Psammechinus, Lyte- 

 chinus. Echinus, Parechinus, Nudechinus, Evechinus, Toxopneustes, Tri- 

 pneusies and Gymnechinus; according to my view only Psammechinus, 

 Echinus and Parechinus belong to this family, Evechinus being an Echino- 

 metrid, while the rest of them belong to the Toxopneustidae. No larvae 

 belonging to the genera Parechinus (in the sense of Clark, which I adopt), 

 Nudechinus or Gymnechinus are known, while of the other genera we know 

 quite a fair number of larvae, so that we can see whether they favour 

 Clark's views or those of the present author. 



The two species known of the genus Psammechinus, milians and micro- 

 tuberculatus, have both been studied as regards their development, miliaris 

 being very completely known, while there is no description of the fully 

 formed larva of microtuberculatus. The larva is characterized by having, 

 in the first stage, long body rods, widening in the end; no basket structure. 

 In the second stage the larva has four epaulets, situated at the base of 

 the four main arms. There is no posterior transverse rod or posterolateral 

 processes. 



