Idem — Revision of the Palaeozoic Palaeechinoidea. 7 



of synonyms by describing Eocidaris hallianus as Archaeo- 

 cidaris hallianus on the strength of some plates he found at 

 Kansas City, Mo., but, concerning whose identity he is not 

 very sure, as he adds: " If the identification of the Kansas 

 Citv specimen is correct, it seems probable that this form 

 should more properly come under Archaeocidaris than Eoci- 

 daris." * Why not leave species where placed until positive 

 of their proper place? 



If writers would keep in mind, that, in all probability, the 

 laws of distribution in geological ages were about as they 

 are at present, they would not describe fifty or more species 

 of one genus from one and the same locality. If we look at 

 the distribution of plants and animals to-day, we find only a 

 very few species of one genus occurring at the same place. 

 Why should we not assume the most logical thing, namely, 

 that the same laws existed in the geological ages, and, when 

 we find only slight differences in our specimens, ascribe them 

 to abnornal development, at least, until we find that the fea- 

 ture first noticed, predominates in a large collection from the 

 same locality or from the same geological horizon. 



Another feature, that should never be lost sight of, is the 

 probable condition under which the animal in question lived 

 and thrived. Just as living plants and animals develop more 

 rapidly under favorable conditions — such as plenty of light, 

 air and food — so, undoubtedly, the rapidity of development 

 in plants and animals during geological times was influenced 

 to a high degree by the surrounding conditions. No doubt, a 

 Melonite with an insufficient supply of food, light and air, 

 secreted only small plates, while under more favorable condi- 

 tions, the deposits would have been more rapid, and, conse- 

 quently, the plates larger. The specimen figured on PI. IV., 

 Fig. 8,- is the same species as the one on PL V., Fig. 9, 

 although in the latter the plates are twice as large. 



As we find a great variation in the size of the individual 

 plates, we must necessarily fiind as great a variation in the 

 number of plates composing a column and also in the number 

 of rows. It is not an uncommon thing to find a great differ- 



* Missouri Geological Survey, vol. 4, p. 129. 



