228 ECHINODERMATA— PELMATOZOA phylum iv 



to different groups of Paleozoic Crinoids, and by which they were differentiated, have 

 become fixed and generally constant in this. It is by the combination of a number 

 of well-marked characters, therefore, that the definition of this group, as herein given, 

 becomes logically effective. 



The results obtained during recent years from the study of the Crinoids collected 

 by a large number of deep-sea expeditions, have thrown an entirely new light upon 

 the relative importance of the Recent and fossil forms, and have shown that there exists 

 to-day a wealth of generic and specific types hitherto quite unexpected. In order to 

 call attention to the relative importance of the fossil and recent types, and to bring to 

 the notice of paleontologists the work which has been accomplished on the latter, it 

 has seemed advisable to include mention herein of a considerable number of Recent 

 genera. As the paleontologist is most directly concerned with the stalked genera 

 among living forms, short definitions of these are given ; the unstalked living genera, 

 which are much more numerous, are mentioned by name only. 



The Pentacrinids and the Comatulids form two groups which are in every way 

 strictly parallel, and are of substantially the same phylogenetic value, though depart- 

 ing in exactly opposite directions from the parent stock. The Pentacrinids are 

 characterised by excessive stem growth ; the larval stem is lost at a very early age, 

 but new columnals are continuously formed, with great rapidity, so that a stem of 

 enormous length results. The distal portion of this stem is continually dying away, 

 so that the actual length of the stem in any individual is but a fraction of the entire 

 length which has been formed during growth. In living Comatulids the larval stem is 

 similarly lost ; but after this takes place no additional columnals are formed ; stem 

 growth continues within the single columnal which remains attached to the calyx ; 

 this becomes greatly enlarged, and puts forth numerous cirri. Comatulids may 

 therefore be described as Pentacrinids in which the entire stem is reduced or limited to 

 the compass of a single columnal, and in which the cirri (when present), unable to 

 arrange themselves in whorls on regularly spaced nodals, are closely packed together 

 on a single nodal. 



The genus Thiolliericrinus is exactly intermediate between the Pentacrinids and 

 the Comatulids ; the stem is developed just to the point at which the two groups 

 diverge, at that point ceasing further .growth, as in the Comatulids, but being retained 

 as in the Pentacrinids. The structure of the stem is the same as that of the larval 

 stem of the Pentacrinids and of the Comatulids. 



The Pentacrinids and Comatulids are the dominant Crinoid forms in the modern 

 fauna. The latter especially are extremely numerous, and exist in a vast array of 

 diverse types, none of which, however, depart in any great degree from the general 

 structure of the grouji ; so that their classification necessitates the creation of numerous 

 subfamilies, families and higher groups which are not systematically comparable to 

 similar groups in the stalked forms.. 



In order that the treatment herein adopted may be more easily understood, the 

 following comparative table is given, which shows in heavy-faced type the names 

 employed by P. H. Carjoenter in the Challenger reports and largely used by paleon- 

 tologists, together with their modern equivalents. 



1. Pentacrinus : Isocrinus, Ce§ocrinus, Endoxocrinus, Hypalocrinus, Carpenterocrinus. 



2. Extracrinus : Pentacrinus. 



3. Antedon : All the genera wliich were known to Carpenter now included in the families 

 Zygonictridae (excepting Eudiocrinus), Himerometridae, Stephanometridae, Pontiometridae, 

 Mariainetridae, Colobemetridae, Tropiometridae, Calometridae, Thalassometridae, Charito- 

 metridae and Antedonidae (excepting Promacliocrinus). 



4. Actinometra : All genera included in the Comasteridae. 



5. Eudiocrinus : The genus Eudiocriniis of the Zygometridae, together with Pentametro- 

 criniis of the rentanietrocriaidae. 



6. Promachocrinus : The genus Promachocrinus of the Antedonidae, together with 

 Decametrocrinus of the Pentametrocrinidae. 



