580 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The columnal immediately following the centrodorsal is always attached to the 

 latter by a so-called stem syzygy. It may be either of nearty the same or of lesser 

 diameter than the centrodorsal, as in 



Comactinia meridionalis. Antedon petosus. 



Comissia littoralis. Antedon bifida, 



fLamprometra protectus. Antedon mediterranea. 



Ptilometra mulleri. Antedon adriatica, 



Crotalometra porrecta. [fHeliometra glacialis~\. 



Glyptometra tuberosa. Anthometra adriani. 



Thauma.tometra nutria-: 

 or it maj' be broader than the centrodorsal, as in 



Promachocrinus kerguelensis. Hathrometra tenella. 



Hathrometra prolixa. Hathrometra sarsii. 



In the species in which the columnal following the centrodorsal is of greater 

 diameter than the centrodorsal it is always more or less strongly lobate, the lobes 

 being directed interradially. It may remain in the condition of a flat pentalobate 

 plate, or the lobes may thicken and form considerable inclusions in the central 

 portion of the lower margins of the basals, appearing like a second circlet of small 

 basals oriented as the first. 



In all of these species the centrodorsal is conical, tapering to a more or less 

 sharp point. It appears to arise as a minute calcareous ring between the middle 

 of the basal circlet and the following columnal, which, receiving additions only 

 on its proximal side, gradually takes on the form of a cone and grows to a con- 

 siderable size before its increasing length finally separates the following columnal 

 from the basals and reveals it as a small cone, with the base upward, connecting 

 the center of the basal ring with the center of the much broader following columnal. 



The first cirri are formed as soon as the basals become separated from the 

 second columnal, and this led Professor Sars to state that in Hathrometra sarsii 

 the first cirri arise from a slit between the calyx and the topmost (in reality the 

 second) columnal. 



The appearance of the proximale, always firmly attached to the calyx by 

 close suture, indicates the maturity of the column in the group of crinoids, the 

 Articulata, to which all the recent forms belong, with the exception of the Plicato- 

 crinidse. The proximale may be larger than the following columnals, as in Apio- 

 crinus and Bathycrinus, of the same diameter, as in Phrynocrinus, or of lesser 

 diameter, as in Rhizocrinus. 



From the contrasting conditions in Bathycrinus and Rhizocrinus it is evident 

 that in the Bourgueticrinidse we have conditions parallel to those in the comatulids, 

 in some genera the proximale (the homologue of the centrodorsal) being larger, 

 and in others smaller, than the following columnal. 



Terminal stem plate. In the pentacrinoid larvae of the comatulids the ter- 

 minal stem plate takes a number of more or less distinct forms. In the very young 

 it is always circular ; from this circular type a lobate form develops, and the lobes 

 may elongate into five regular tapering processes, or may develop very irregularly 

 into a group of digitiform extensions which are in reality radicular cirri, and which 



