490 FRANK SPRINGER 



three primibrachs. I have never seen a specimen with only one, 

 although it might happen as a mere abnormality; but the case is a 

 most interesting one upon the actual facts. This species, from the 

 Burlington Limestone, systematically and regularly departs from the 

 brachial arrangement of all other described Forhesiocrini, in having 

 but two primibrachs. It is a rare species, and specimens are espe- 

 cially rare in which all rays are exposed; but usually at least three 

 are visible, enabling us to see the prevailing type. I have thirteen 

 specimens, and have observed seven in other collections, all of which 

 show two primibrachs in the visible rays. There is another form 

 occurring still more rarely in the Burhngton Limestone which has 

 the normal arrangement of three primibrachs. It is considerably 

 smaller than F. agassizi, but strongly resembles it, and has hitherto 

 been assumed to belong to it. It might be treated as of the same 

 species, which would thus exceptionally embrace representatives of 

 structures elsewhere widely distinct; or, what is perhaps the more 

 rational view, good reason may be given for separating F. agassizi, 

 both specifically and generically. 



In Taxocrinus also there is some irregularity of this kind among 

 certain Devonian species; that is, some individuals have two primi- 

 brachs, while others of apparently the same species have three. In 

 a specimen of Taxocrinus intermedius from the Kinderhook two rays 

 are irregular, and in the type of T. nobilis from the Mountain Lime- 

 stone, one. Here the Devonian and Lower Carboniferous may be 

 considered the transition period, in vfhich the three primibrachs were 

 being established in the genus. There is also to be seen in occasional 

 specimens in the Kaskaskia a tendency toward reversion to the two- 

 primibrach structure. 



Ichihyocrinus greenei, of the Keokuk Limestone, is the only exam- 

 ple of Ichihyocrinus or its carboniferous representative with three 

 IBr in more than one ray. It is known only from a single specimen, 

 which does not expose the anal side; and w^e cannot be certain of 

 its generic relations, notwithstanding its close superficial resem- 

 blance to the typical Ichthyocrini. 



I am not concerning myself, however, with the few exceptional cases 

 which may be found; but if anyone will examine such a series of the 

 four leading genera above mentioned as I have before me, with hun- 



