268 SMITHSONIAN INSTITUTION 



Such was the state of opinion when I discovered and pointed out in 1906 * that in all the 

 Silurian species of this genus there is an extra plate in the right posterior ray situated directly 

 underneath the radial, and identified this plate as the Radianal in its primitive position as an 

 infer-radial, just as it had been found in the Taxocrinoid genus Temnocrinus, and in the 

 Inadunate genus Dendrocrinus. 



This discovery made it necessary to remove the Carboniferous species, in which the 

 radianal was found to be absent, under the name Metichthyocrinits, thus leaving Ichthyo- 

 crinus, so far as then known, a purely Silurian genus. 



The history of this genus is an instructive example showing how, under the influence of 

 prevalent notions and modes of interpretation of fossils, palpable facts having a most impor- 

 tant bearing upon their zoological relations may be overlooked or ignored throughout a 

 series of apparently careful researches by conscientious naturalists extending over half a 

 century. The existence of the extra plate in one ray has been plainly evident ever since Hall 

 published his fine figures of I. laevis in 1852 ; but from the point of view from which paleon- 

 tologists considered it this fact had no significance, and was treated as a mere irregularity. 

 It comes to the front now as the firm basis for a much desired separation between the 

 Silurian and Carboniferous species, which I think must gain the approval of every careful 

 student of the crinoids. 



Throughout all the shifting of opinion just recited touching the definition of the genus, 

 there is one reliable fact, viz. that we know to a certainty the form upon which Conrad 

 founded the genus, which he figured as his- typical species, and which his contemporary, Hall, 

 had in mind when he brought together under it the American and British species as identical. 

 And it is a satisfaction to find, as the final outcome of these various changes, that we are led 

 by a plain and evident road back to the original species as the sure and unmistakable founda- 

 tion of the genus. 



The most characteristic feature of Ichthyocrinus is the complete absence of interbrachial 

 structures, either anal or otherwise. This fact, together with the generally small size of the 

 basals, produces a rapid increase in the width of the primibrachs from the radials up. There 

 is nothing that can properly be called a calyx such as is recognized in crinoids generally. 

 The basals, even when the largest, are not entirely free from the stem, and the radials are 

 much smaller than the succeeding primibrachs, being sometimes also partly covered by the 

 stem. The main constructive energy of this crinoid, so far as can be seen from the dorsal 

 side, seems to have been directed toward the brachial system, which has an uninterrupted 

 sweep from the very base of the crown to its summit, giving to the fossils of the genus a 

 habitus so peculiar that it can be recognized from the smallest fragment of the calyx. This 

 peculiarity is shared by Parichthyocrimts and Metichthyocrimis from the Carboniferous, so 

 that a view of the posterior ray, or a knowledge of their geological position, is necessary to 

 distinguish them. Clidochirus has something of the same aspect, but less pronounced by 

 reason of the larger basals and the presence of an anal plate ; and it prevails to a greater or 

 less extent throughout the family Ichthyocrinidae. 



The interlocking of the rays and their divisions is also a very marked character of this 

 genus. Beginning with the primibrachs there is a slight irregularity in the height of the 

 corresponding plates of adjacent rays, so that one suture stands at a little higher level than 

 the opposite one. Thus the plates of the same ranges alternate somewhat with each other, 

 and the vertical lines of junction between the rays and their subdivisions are not straight, but 

 somewhat zig-zag. This structure produces a firm interlocking of all these plates up to and 

 including most of the third order, beyond which the arms gradually become free, though 

 usually still abutting. The height to which the interlocking continues varies with individual 

 growth, and perhaps with different species ; it is less in young individuals than in those of 

 mature age. There is, however, no point where the arms can be said regularly to become 



1 Journal of Geology, vol. 14, pp. 475 et scq. 



