20 IH l.l.i: l I N B2, I Ml IH) STATES NATIONAL MUSEUM 



7 j . I \ Br . 69°; IVHr,, 70°; IVBr 7 , 64°; [VBr 8l 69°; IVBr,,, 69°; IVBr 10 , 76°; brachials 

 1. 7 :., 72°; G, 7:i°; 7, (39°; distal brachials, 72 75°. 



From these figures il is evidenl thai as a rule obliquity of the muscular articula- 

 tions i- ni"-! stronglj pronounced immediately following the axillaries, ami that the 

 more distal articulations become less oblique, ami later in the distal portions of the 

 arms, again somewhat more oblique. An exception to this is found in Rycrinus 

 carpenterii in which the distal angle of the axillary (as viewed dorsally) is very large. 

 Here the proximal brachials, which also lack pinnules, are only slightly oblique, 

 whether viewed from the dorsal side or from the articular face. 



As a rule in the comatulids practically without exception judging from 20 speci- 

 mens examined in addition to those listed above and agreeing with them — the oblique 

 muscular articulations show the following sequence: After the first brachial articula- 

 tions at the base of tin- arm they tend toward the straight type. Gislexi's figures of 

 the twenty-seventh brachial of HeUometra glacialis and of the thirteenth brachial of 

 Heterometra cnnulata show how close the oblique muscular articulation on certain 

 proximal brachials can come to the articulation on the radial articular face. This 

 straight type, according to Gislen, occurs more or less completely developed after a 

 varying number of brachials. It should be noted, however, that discoidal brachials, 

 that is, brachials with externally straight articular lines, appear before the tenth 

 brachial whereas the interior straightness of the articulations does not generally 

 appear until after the tenth or fifteenth brachial. These two types are therefore 

 usually separate, and seldom appear on the same brachial. 



Gislen remarked that Clarkometra elegans shows, for an antedonid [I consider it 

 a colobometrid], a rather high figure for the angle on the second brachial. He sup- 

 posed that on., might imagine this to be caused by the lack of pinnules; but the fourth 

 brachial, which is also nonpinnulate, shows a normal figure. 



That inner obliqueness can be extraordinarily strong is shown by Pentametro- 

 crinus, in which the first pinnule appears on the fifth brachial. The species of Atrfe- 

 rrinus lack pinnules to about the fifteenth brachial. In spite of this, external oblique- 



18 well developed. Inner obliqueness, on the other hand, is very little marked, 

 and straight articulations are met with already at about the sixth brachial. 



Gislen inferred one can understand from this that in the comatulids the presence 

 or absence of pinnules plays a rather small part in regard to external obliqueness in 

 'lie proximal portion of the arms. It is of more importance in regard to inner oblique- 

 alt hough it is obvious that in the more proximal parts of the arms other factors 

 appear that influence the obliqueness of the muscular articulations. The low figures 

 of the angles in Eudiocr'mux may be caused by the fairly thick pinnule bases. 



If the' morphological reasons for the oblique angle between the intermuscular 

 fossa and the transverse ridge are studied the following facts appear: The difference in 

 size between the two muscular fossae is usually of less importance; the difference in 

 size between the interarticular ligament of the pinnular side and that of the anti- 

 pmnular side is, however, often much greater. This difference reaches its maximum 

 in the antedonids. Through the interaction of muscle, pinnule socket, and interartic- 

 ular ligament the pinnular side of the distal facet of the segment is enlarged ventral 

 to the transverse ridge, and as a result the dorsoventral intermuscular furrow looks as 

 if it were forced over toward the antipinnular side. In reality, however, this is not 



