TAXOCRINIDAE 



431 



in length of the main ray divisions or rami. The dimensions of these in representative 

 mature specimens of the three species and O. asteriaeformis are: 



0. asteriaeformis 



O. diversus 



0. ulrichi 



0. exsculptus . . . 



Ray 



25 



35 

 40 



45 



Ramus 



10 

 20 

 3D 

 56 



Diam. of calyx 



19 

 30 

 38 



35 



Base 



9 



10.5 



Thus it appears that while in the two earlier species the ramus was from 60 to 43 per 

 cent shorter than the ray, in this one it is 25 per cent longer ; with this increase in length 

 the rami are proportionally more attenuate, and the clusters of ramules much more numer- 

 ous — being in mature specimens from 13 to 15 as against 7 to 9 in 0. ulrichi, and 3 to 5 

 in 0. diversus. Thus in all these related characters there has been a progressive increase in 

 the direction of elongation of the rami. The much greater delicacy of the ramules will dis- 

 tinguish this species in a mere fragment, and also in the young where the other differences 

 are not so marked. The species also attains a larger size than the others, a maximum 

 specimen being 125 mm. in total length of crown, against 70 mm. in 0. ulrichi. 



In this species also the number of IBr is substantially constant at 4, the variation being 

 about five per cent in upward of 100 specimens ; among these is an occasional ray with 

 5 IBr, though most of the variation is to 3. Plate LXX contains a selection of figures of 

 specimens from the same colony in various stages of growth, by which all these characters 

 are beautifully illustrated. And on Plate LXXI, I have given a figure of two specimens just 

 as they lie on the slab together, one showing the stem studded with small cirri toward the 

 end, and the other having the stem entirely complete, with its strong cirri at the root. These 

 figures, with a scale to correspond, are reduced to about half the natural size ; so it can be 

 seen that the complete stem is about 22 inches long. 



The ventral structure is the same as has been described for 0. ulrichi, but is very rarely 

 seen, notwithstanding the abundance of specimens at the typical locality. Figures 3, 4, 5 

 of Plate LXIX represent all of it that has ever been discovered. Figure 3 is from one of 

 Lyon's specimens, upon which he made an excellent description of the vault, to the extent 

 of identifying correctly the ambulacra, and the small irregular granulose " interstitial pieces 

 lying in the fields bounded by the five rays," which he says " are composed of two rows of 

 large granular pieces, one now alternating with the other " (Amer. Jour. Science, vol. 29, 

 i860, p. 79). Figure 5 shows all the ventral structures except the small orals, which are 

 doubtless buried in the perisome ; it also gives an instructive view of the clusters of ramules, 

 showing how they diminish in the number of ultimate arms from 10 to 12 in the lower, to 

 only 2 in the upper; six clusters are visible on the inside of the long ramus at the lower side, 

 and there were one or two more, which would make at least 15 clusters for both sides of the 

 ramus. 



It was upon this species that Colonel S. S. Lyon founded the genus Onychocrinus in 

 i860. He recognized and correctly described its essential characters, viz., the long rays, 

 with two gradually tapering; branches fringed on either side with alternating clusters of 

 what he called " pinnules." The widely extended rays, together with the traces of radiating 

 ambulacra in the vault, impressed him strongly as forming a probable connecting link between 

 the crinoids and star-fishes. 



Lyon was a very ardent student of the crinoids. Most of his descriptions, including 

 those under this genus, were not illustrated, from lack of means to procure the printing of 

 the necessary plates. He prepared, however, excellent drawings of all his type specimens, 

 and arranged them upon twenty-three plates, intending to bring out a monograph of the 



