A MONOGRAPH OF THE EXISTING CRINOIDS 21 



entirely so. If one studies the ventral side of an arm from which all the soft parts 

 excepting the ligaments have been removed, one will find that the intermuscular fur- 

 row or crest is always oriented strictly dorsoventrally. It is therefore more correct 

 to express the inner obliqueness of the articulation by saying that muscle, pinnule 

 socket, and interarticular ligament displace the transverse ridge toward the dorsal 

 side of the segment, or the pinnular side of the distal end of the ossicle — or the anti- 

 pinnular side if the ossicle is observed from the proximal end. 



Gislen examined the distal articidar faces of the axillaries to determine whether 

 they should be interpreted some as straight and others as oblique, or as intermediate 

 between the two types. The following figures were obtained by measuring the distal 

 axillary angles (the lesser angle between the transverse ridge and the dorsoventral 

 crest). 



Comanthus japonica: (IBr t proximal 87°; IIB^ proximal 78°); IIBr axillary 

 67-70°; IIIBr axillary 60°. 



Himerometra magnipinna: IIBr axillary 50°; IIIBr axdlary 63°. 



Pontiometra andersoni: IBr axillary 87°; IIIBr axillary 82-85°; IVBr axdlary 

 78-82°; VBr axillary 80°. 



Tropiometra ajra macrodiscus: IBr axdlary 80°. 



Cenometra bella: IIIBr axillary 76°. 



Oligometrides adeonae: IBr axillary 77°. 



Pedinometra fiavopurpurea: IBr and IIBr axillaries 90°. 



Asterometra anthus: IBr axillary 87°. 



Notocrinus virilis: IBr axdlary 87°. 



Clarkometra elegans: IBr axillary 84°, 84°, 85°, 89°. 



Heliometra glacialis: IBr axillary 86°. 



Hycrinus carpenterii: IBr axillary 74°. 



Metacrinus nobilis tenuis: Radials 89°; IBr axillaries (4) 70°; IIBr axillaries (7) 

 63°; IIIBr axillaries (11) 67°; IVBr axillaries (11) 68°. 



From this it appears that Comanthus, Himerometra, (Pontiometra), Tropiometra, 

 Cenometra, Oligometrides, Hycrinus, and Metacrinus have oblique articulations, while 

 the other types examined have nearly straight articulations. 



It is evident from these figures, according to Gislen, that no sharply defined 

 boundary can be found between straight and oblique muscular articulations. 



Gislen said that if one accepts the later of the standpoints taken by the author 

 one should be able to maintain that the straight muscular articulation on the distal 

 face of the radial is always single, while all the other ossicles with muscular articula- 

 tions have double articular faces, that is, form axillaries or bear an arm and a pinnule 

 on their distal ends — though in this connection one must remember that there are 

 types with defective pinnule tion. In such a case Zi and Z 2 would always correspond 

 to the first two brachials which might also, with a certain degree of justice, be called 

 homologous in the recent crinoids. Gislen concluded it can be stated that the homologi- 

 zation of certain proximal brachials based upon a supposed distinct difference between 

 straight and oblique muscular articulations is erroneous. 



Gislen noted that the arm divisions are brought closer together by the first two 

 brachials being longer on the outer side of the arm than on the inner. This is of some 

 importance, for after the first arm division the arms in 5-rayed forms cannot, if they 



