NO. 1937. CRINOIDS OF THE BERLIN MUSEUM— CLARK. 407 



ceptibly as is the case in the other species of the genus; this I find to 

 be quite a characteristic feature. 



In this species also the distal intersyzygial interval is usually four 

 oblique muscular articulations instead of three as in the other forms. 

 The greater size of the species accounts for this, however, as among 

 the comatulids the distal intersyzygial interval is determined by the 

 size of the animal and the number of arms, having only a remote and 

 purely secondary relationship with generic or specific characters. 



In fully grown specimens of this species the cirrus sockets near the 

 periphery of the centrodorsal are arranged in four regular columns; 

 of these the two outer consist of five or six sockets, but the two cen- 

 tral have two (in smaller specimens one) sockets only, being distally 

 replaced by a single column of three sockets, so that whereas in the 

 proximal part of the centrodorsal there are four columns of cirrus 

 sockets in each radial area in the distal there are but three. Young 

 specimens of course have three columns of cirrus sockets in each ra- 

 dial area until the peripheral diameter of the centrodorsal is equal to 

 the diameter through the lower sockets of the central columns in the 

 centrodorsal of the adults. 



Young specimens of this species, like young specimens of Heliometra 

 glacialis, have the borders of the ossicles of the IBr series and lower 

 brachials prominently spinous. 



Carpenter and other authors have recorded Hathrometra teneUa as 

 occurring in the Arctic regions ; but I have never been able to exam- 

 ine specimens of any species but prolixa from the Arctic, and I have a 

 strong suspicion that the so-called tenella is merely the young of 'pro- 

 lixa, or is based upon specimens which have lost their long and char- 

 acteristic cirri. H. tenella is confined entirely to the western side of 

 the Atlantic, and does not extend very far to the northward, not 

 intruding at all upon the territory occupied hj prolixa. 



Though useless for ordinary systematic work the small subperiph- 

 eral and polar cirri have a very great value in tracing out the line 

 of descent of the species in which they occur; for each of these small 

 cirri represents a fully grown cirrus at the stage in which the border 

 of the centrodorsal was just beyond the proximal edge of its socket. 

 The centrodorsal increases by addition to its proximal edge; when 

 enough material has been added cirri are protruded between the 

 proximal row of cirri and the border of the centrodorsal. These 

 cirri grow to a certain length and attain certain definite characters. 

 The cirri of the next row above grow to a greater length and possess 

 a greater number of segments. This process keeps on until maturity 

 is reached, after which the cirri of subsequent rows show no pro- 

 gressive change. The small polar and subperipheral cirri, the 

 "small mature" cirri of Carpenter, are really persistent relics of 

 successive young stages, and have no connection with the adult 

 stage at all. 



