CRINOID GENUS SCYPHOCRINUS 35 



lars diminishing in size and numbers upward; but their more distal parts, and 

 the whole length of those succeeding them, lie side by side, closely packed, 

 forming part of the calyx wall, until they gradually lose themselves in the 

 tegmen. This fixation of pinnules extends throughout the tertibrachs to the 

 quartibrachs, passing gradually into a free state, the exact limit of which 

 cannot be seen for lack of sufficient preservation of the tegmen. All these 

 pinnules are laterally united by what is probably a loose suture, and the plates 

 composing them, as high up as the distal tertibrachs, are very thick and flat, 

 forming by their union a strong wall. The proximal pinnular is joined to the 

 brachial which supports it, and the following pinnulars to each other by a 

 curved, non-muscular articulation, the remnant of the movable union lost by 

 fixation. 



The free pinnules continue thus alternately, one from each wider end of 

 the brachials to the end of the arms; they are relatively short, as is the rule 

 generally in multibrachiate arms, rounded, composed of a few elongate pinnu- 

 lars; and they lie closely abutting, because of the extreme shortness of the 

 brachials; distally they become rounded, less closely abutting, and functioning 

 normally. The relation of these parts is shown on Plate IV by the enlarged 

 drawings at figures 4 to 10, with further explanatory text. 



Thus the law of succession of the pinnules on the secundibrachs is : 

 2(1+2) outer — 4(3+4) inner — 5 outer — 6 inner — 7 outer; and so on. 

 It may be expressed by the numeral 24567+. 



This arrangement appears in several of the other species, and is substan- 

 tially the same as in Melocrinidse, generally where it can be observed. It is 

 finely shown in the specimen of Glyptocrimis dyeri figured in my paper on 

 Cleiocrinus, which I reproduce for comparison (PI. Ill, figs. 5a, b). The suc- 

 cession of pinnules is the same, 2-4-5-6-7, etc., and their course is much more 

 plainly defined by the sharp longitudinal ridges. In this specimen the tegmen, 

 although distorted at one side by an imbedded stem-fragment, is well preserved, 

 and shows how the fixed pinnules merge into the finely plated perisome, which 

 passes into the tegmen so gradually that it is impossible to distinguish a 

 boundary at which the pinnules should become free. In a similar manner the 

 ridge-like series of anal plates is seen bending over, probably to a small central 

 opening. The same plan of tegmen structure no doubt existed in Scyphocrinus, 

 in which this part has not yet been found intact. 



The sequence above described is also precisely the same as that of the 

 fixed pinnules in Uintacrinus, except that in the latter the plates 6 and 7 form 

 a syzygial pair, the pinnule being borne only on the hypozygal there as well as 

 in the higher parts of the arm. In Scyphocrinus there are no syzygies, 

 although it is possible that in the primitive form of these crinoids, in the inter- 

 mediate stage without interbrachial structures and with free arms beginning 



