188 R. T. JACKSON — STUDIES OF PAL^ECHIXOIDEA. 



The species Oligoiporus coreyi differs from OUgoporus danx as described 

 in the original publication, which is here confirmed, in being much 

 smaller and apparently more depressed in form ; in having more deeply 

 furrowed ambulacral areas,* and also only 6 columns of interambulacral 

 plates. From OUgoporus nobilis it differs by its smaller size and more 

 deeply sulcate areas, which are proportionately wider. OUgoporus coreyi 

 differs from OUgoporus mutatus, Keyes (23), and 0. missouriensis, Jackson, 

 in having relatively much smaller plates and in being much smaller 

 as a whole ; also in being a fine delicate species rather than robust in its 

 proportions ; it also has much less of a melon-like rotundity in its areas 

 than 0. mutatus, as well as one more column of interambulacral plates 

 than that species. 



It is shown in echinoids that, as the individual grows, new columns of 

 interambulacral plates are progressively added until the number normal 

 to the species is attained. It may, therefore, be properly questioned 

 whether this specimen is an adult, and not the young of another species 

 as OUgoporus danx, which in later growth, had the animal lived, would 

 have added more columns of plates, and the plates themselves have in- 

 creased in size. In many cases this might be a difhcult question to decide, 

 and it is possible that species of Pala^echinoids have been based on im- 

 mature specimens; but from the ver}- principles of growth involved we 

 find the answer in this case. The first 6 columns of plates are here, as 

 in all other cases studied, introduced comparatively earW in the life of 

 the individual, in this case the sixth originating in what is probably the 

 ninth row from the oral end. A seventh row, if it were to be added, 

 should have followed soon after the sixth, as in OUgoporus danee (plate 6, 

 figure 34). In OUgoporus coreyi 6 rows of plates are added after the intro- 

 duction of the sixth column without the a})pearance of a seventh column ; 

 therefore from a comparative study of other related species and genera 

 it can properly be assumed that no more would have been added. 



ARRANGEMENT AND DEVELOPMENT OF PLATES IN OLIGOPORUS COREYI. 



Turning from the consideration of species characters to that of the 

 arrangement and development of the ambulacral and interambulacral 

 areas of OUgoporus, as shown in the specimen of 0. coreyi. The study of 

 OUgoporus and the comparison of its plate arrangement and development 

 with that of Melonites is important on account of the evident affiliation 

 of the genera. Adult OUgoporus is characterized by possessing 4 columns 

 of ambulacral plates, and sometimes accessory intercalated plates, as 

 described in OUgoporus danas. In the interambulacra of OUgoporus there 



*The degree of depression is not shown in our specimen, as it is preserved only as flattened out 

 portions of the test. 



