G02 Mr. H. L. Clark on the 



in position (this is assumed for Anomolanthus , as neither the 

 describer of the unique type species nor the zoologist who 

 established the genus refer to this most important character), 

 there are five genital glands and pores, only slightly petaloid 

 ambulacra, and small paired interambulacral plates in contact 

 ■with each genital. In Clypeaster the interambulacra may 

 become discontinuous in the adults, i. e. the primordial inter- 

 ambulacral plate becomes separated by ambulacral plates 

 from the rest of the interambulacrum. It would be very 

 interesting to know whether this is also true of Anomolanthus, 

 for the high test and marginal anus of that genus show a 

 less degree of specialization than is shown by any Recent 

 Clypeaster. 



With the increasing tendency to a subarenaceous life the 

 depressionof the test has gone on rapidly in the Clypeastroida, 

 the extreme being reached in Arachnoides placenta. Curiously 

 enough, however, except for the discontinuous interambu- 

 lacra, doubtless associated with the flatness of the test, 

 Arachnoides is otherwise quite a primitive form, as evidenced 

 by the paired ambulacral auricles and the supra-marginal 

 anus. The ambulacra, too, are only slightly petaloid. How 

 the real relationships of this remarkable clypeastroid can best 

 be shown is, of course, debatable, but it seems to be contrary 

 to the evidence to separate it widely from Clypeaster, and yet 

 I cannot believe it ought to be placed in the same family with 

 that genus. I therefore recognize two distinct but nearly 

 related families, the Clypeastridse and the Arachnoididse. 



While Arachnoides is thus superficially very unlike Clype- 

 aster but really nearly related, Layanum represents the other 

 extreme, for it is superficially much like Clypeaster but in 

 reality is qaite different. In the fused auricles, interradially 

 placed, and in the very narrow continuous interambulacra 

 Avith a large and apparently (though not really) unpaired 

 abactinal plate adjoining the genital, Laganum shows struc- 

 tural progress that makes it improper to place it in the same 

 family Avith Clypeaster. The peculiar character of the inter- 

 ambulacra, combined with the straight, simple, actinal 

 ambulacral furrows, prevents our placing it in the same 

 family with Scutella and its allies, and it is therefore necessary 

 to recognize the family Laganidse. 



The remainder of the clypeastroids form quite a homo- 

 geneous group with more or less discoidal tests, fused 

 auricles, interradially placed, interambulacra beginning 

 abactinally in a pair of small plates next the genital, and 

 with actinal ambulacral furrows more or less forked or 



