164: Report of the State Geologist. 



various series established by taking into account only the varia- 

 tion of a given structure, do not often fuse into a single series, 

 as the principle of correlation would require. This fact leads 

 us to a new conception of great importance. 



We say that an animal of a certain group is aberrant as to one 

 of its organs, when this organ, through its structure, cannot be 

 admitted into any of the morphologic series constituted for homo-^ 

 logons organs in the group in question. It is best to restrict, as 

 we have done, this term, which is somewhat misused ; the evolu- 

 tion of an organ can take place in divers directions, and we are 

 not to consider as aberrant a series which, though . less extended 

 than another, may be quite as normal. 



Of the forms which are well represented in a fossil state, we 

 may cite among Crinoids the genera Barrandeocrinus, Eucalyp- 

 tocrinus; among the Echini, the Dysasteridae ; among the Mol- 

 lusks the Teredinae, the Rudistae^ the Trigoniidae, the AnomiaB. 

 The Arthropoda will furnish the Limuli ; the Fishes, numerous 

 types as the trunk-fishes, the genus Amphysilene, etc. Among 

 Reptiles we find Triceratops; among the Mammalia, Dinoceras and 

 many others. Comparative anatomy shows many examples of 

 animals which by nearly all their characteristics are naturally 

 ranged in a determinate series, but which in one or more organs 

 differ widely from the forms nearest to them. 



Among these aberrant types the most interesting are those 

 which present in association the characters of several distinct 

 groups, without on that account taking a place precisely intermedi- 

 ate between any two of these groups. The fossil Echinoderms 

 present very instructive examples. The exclasively palaeozoic class 

 of the Cystidiae is a polymorphic group, which presents instances 

 of transition more or less distinct with the Asterias, Echinoids, 

 Crinoids and Blastoids. These four classes, oil the contrary, are 

 very clearly defined among themselves, and it is almost impossible 

 to maintain that they are derived one from another. Bat there 

 exists a curious type, Tiarechinus^ which presents at once the 

 characteristics both of the Blastoids and the Echinoids. This 

 type, entirely isolated, is limited to the Trias ; that is to say, it 

 appears long after the extinction of the Blastoids and after the 

 type of the Echinoids has undergone an important evolution. It 

 is a synthetic type of the most singular kind. 



