A MONOGRAPH OF THE EXISTING CRINOIDS 43 



Ptilocrinida — Coiil iniied . 



Second group; Hyocrinoida — 



Ptilocrinidae (Ptilocrinus; Calamocrinus) . 

 Hyocrinidae (Hyocrinus; Gephyrocrinus) . 



Comatulida : 



Pentacrinitidae. 

 Comatulida. 



Hyocrinus is assumed to possess the simplest type of column. 



The comatulids and pentacrinites are considered to be on essentially the same 

 phylogenetic plane; in the former the column is suppressed, while in the latter it 

 undergoes excessive development. 



The reduction of the calyx plates in the comatulids and pentacrinites from their 

 primitive status as elements of a capsule inclosing the visceral mass to the status 

 of a platform consisting of three alternating circlets of five plates each superposed 

 upon each other is explained. It is pointed out that a parallel metamorphosis has 

 taken place in Bathycrinus, but here the plates have moved directly inward toward 

 the chief axis of the animal forming a column on the distal end of which the visceral 

 mass rests. 



Through Dr. W. Weltner and Dr. R. Hartmeyer, the collections of the Berlin 

 Museum, including such of Muller's types as are in that institution, had been sent 

 to me, and among them I found the specimen described as Alecto purpurea, which was 

 redescribed and figured on May 27. That it is a valid species was shown by a large 

 number of specimens included in the collection of the Australian Museum, which 

 had also been sent to me through the courtesy of Dr. R. Etheridge, jr. 



On June 6 the origin of certain types of crinoid stems was discussed. Attach- 

 ment by a primitive central plate which increases in size (as in Holopus) was assumed 

 to be followed by fracture and a subsequent free existence (as in Edriocrinus) or by 

 the breaking of the stem in so far as its calcareous structure is concerned, the resultant 

 parts maintaining organic continuity, a process which may be indefinitely repeated, 

 resulting in a series of columnals with the fulcral ridges on the two ends alternating 

 in dii-ection; in other words, a column comparable to that of Bhizocrinus , Bathy- 

 crinus, or the young of Antedon. Such a column in its primitive form is limited in 

 size through mechanical considerations ; to counteract these the individual columnals 

 may become greatly shortened (as in Phrynocrinus) , the fulcral ridges on either end 

 of each columnal may depart only slightly in direction (as in Platycrinus) , or the 

 original fulcral ridge may disintegrate, each half breaking up longitudinally and 

 spreading out fan-like, the two figures eventually imiting to form an articular sur- 

 face composed of numerous uniform radiating lines. The columnals of the pen- 

 tacrinites are of the last type, modified by being molded or cast into petaloid sectors 

 by the under surface of the basals against which they are formed. In a footnote 

 mention is made of a new genus, Proisocrinus, in which the lower part of the column 

 resembles that of Calamocrinus, the upper that of Isocrinus. 



On June 7 a new comatulid, Compsometra lacertosa, was described from Australia, 

 and on June 18 the species of Antedon occurring at Trieste was given the name of 

 Antedon adriatica, and various points regarding the other species of the genus were 

 discussed. On August 6 a new genus of stalked crinoid, Proisocrinus, with the type 



