178 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



very broad, but the median ridge is no wider than usual (part 1, fig. 36, p. 75 ; 

 this is only a moderately developed example). Thus a small synarthrial articula- 

 tion occupies the center of a large flat joint face. As the ligament bundles, on 

 account of the broadening of the rim about them, and its close contact all around 

 with the rim of the adjacent ossicle, are not visible exteriorly, and the close union 

 has eliminated the beveling commonly characteristic of synarthrial articulations, 

 causing the line of union to appear perfectly uniform and straight, the crypto- 

 synarthry appears externally merely as a very fine line and is only with difficulty 

 to be distinguished from a syzygy. 



This peculiar type of synarthry, in which the true synarthrial joint face occu- 

 pies only a relatively small part of the entire joint surface, appears to be the result 

 of a cessation of growth of the synarthry at some time considerably before the 

 attainment of adult size by the animal so that, after the synarthry stops increasing 

 in size, the further increase in the size of the joint face as a whole is effected by the 

 gradual broadening of the borders of the ligament fossae, which effectually masks, 

 so far as the external view is concerned, the synarthrial nature of the union. 



For this peculiar type of synarthry, which is characteristic of certain forms 

 in the genus Comatula and appears to be clearly differentiated from the ordinary 

 synarthry and easily distinguishable even by external examination, I have sug- 

 gested the name of cryptosynarthry. 



It is evident that the cryptosynarthry is not very far removed from the 

 loose suture. 



Large specimens of species belonging to other genera of the Comasteridse often 

 have the synarthrial unions so close as scarcely to be recognizable as such externally ; 

 indeed Carpenter himself described Actinometra notata as having "syzygies" 

 between the elements of the division series and placed it next to Comatula rotalaria, 

 when in reality it is identical with his Actinometra stelligera, belonging in a totally 

 different " group " ; but with a little practice there will never be any danger of 

 confusing the true synarthry and the cryptosynarthry. Where the latter is present 

 the two elements united by it appear as a single ossicle, just as do two ossicles united 

 by syzygy. This is never the case with two elements united by a true synarthry. 



MUSCULAR ARTICULATIONS. 



General remarks. 



The muscular articulations differ from the nonmuscular articulations in 

 possessing a pair of muscle bundles attached to their ventral borders and extending 

 for a greater or lesser distance inward along the median line of the joint face toward 

 the central canal. 



The first muscular articulation unites the radial to the proximal element of 

 the IBr series (figs. 1, p. 6, 1118, pi. 20), and subsequently muscular articulations 

 occur between the pairs of ossicles in the division series and between all the 

 brachials in the free undivided arms except those united in syzygial pairs (part 1. 

 fig. 6, p. 63 ; fig. 30, p. 71). 



