114 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



While correct so far as Eudiocrinus indivisus goes, this construction 

 is quite wrong for semperi, japonic as, and atlanticus, as will be shown 

 later. 



In regard to Metacrinus, Doctor Carpenter says that the first post- 

 radial joint "is actually a syzygial joint with a pinnule on the epi- 

 zygal, just as in the simpler Eudiocrinus indivisus, but an axillary 

 appears a few joints farther on and the rays begin to divide." Now, 

 although no definite statement is made, the inference is that he con- 

 siders the two first post-radial joints in Metacrinus to be, as in the 

 case of Eudiocrinus indivisus, homologous with the first two post- 

 radial joints of a ten-armed crinoid. This is erroneous; but had he 

 compared Metacrinus to "Eudiocrinus " v avians, semperi, japonicus, 

 or atlanticus it would have been correct, as will appear later. 



Of the remaining recent genera (as then known) he says: 



In the other Pentacrinidse, however, in Bathycrinus, Holopus, and in most 

 Comatulse, as well as in the fossil Encrinus and Apiocrinidce, the second joints 

 above the primary radials are axillaries, and it is not till the second (or rarely 

 the first ) joints beyond these that the pinnules appear. In all these types, 

 the axillary and the joint immediately below it are of the same width as the 

 primary radials in the calyx. But in Marsupites and in many Palseocrinoids 

 (Platycrinus, Gyathocrinus, etc.) they are very much smaller than the primary 

 radials, just as the homologous joints are in Hyocrinus. 



The first thing in discussing brachial homologies in the crinoids is 

 to determine upon some method by which we may, with a fair degree 

 of certainty, fix upon single joints, or a pair of joints, as being homo- 

 logous in all the genera and species considered, no matter where we 

 may find them ; when this point is once decided it will be easy enough 

 to work backward and forward from it, and to arrive at the homo- 

 logies of the adjacent parts. Fortunately the determination of such 

 a joint is comparatively simple, when we have a clear understanding 

 of the types of articulation occurring among the recent crinoids of 

 the families under consideration. These fall at once into two groups, 

 muscular articulations, and nonmuscular articulation*, differing, as 

 their name implies, in the presence and absence of muscle bundles. 

 The differences between them may be shortly summarized as follows: 



MtrsctTLAB Articulations (divided Nonmtjsctxlab Articulations (di- 



Lnto ("i straight, and (6) oblique). vided into (a) synarthries or bifascial 



articulation* and (.&) syzygies). 



Muscl,- bundles present. Muscle bundles absent. 



May bear pinnules, or may be Never hear pinnules, and are never 



doubled, thus forming an axillary with doubled, 

 an additional arm. 



Whether pinnulate or nol always af- Have no effect on pinnulation : the 



fects the position of the aext follow- SUCC eeding pinnule occupies exactly 



ing pinnule, throwing it to the oppo- the same position as it would were the 



site side of the arm from the immedi- nonmuscular articulation not there, 



ately preceding pinnule. Dut the two joints connected by it 



merely a single joint. 



a See beyond, under Comastcr and Isocrinus, and also Metacrinus. 



