TRANSACTIONS OF SECTION D. 919 



Dividing all Criiioidea into Monocyclica and Dicyclica, we trace in each Order 

 a gradual and, to some extent, parallel modification, here and there diverging in 

 somewhat similar directions. Thus the simplest forms in each Order are Inadu- 

 nata, with free distinct arms, and pass from a Larviform stage, with simple 

 tegmen, to a Fistulate stage, with more complex anal tube and tegmen. At an 

 early period (? Cambrian) in the histor}' of the Monocyclica, the Camerate modifi- 

 cation — viz., rigid incorporation of brachials in cup and ambulacrals in tegmen — 

 affected a few forms, and thus arose Monocyclica Camerata. At a later period 

 (Silurian) was a repetition of this modification, but one atiecting the cup to a far 

 less extent, and resulting chiefly in a solid tegmen and biserial arms ; thus arose 

 the Monocyclica Adunata (or Platycrinoidea), which even Wachsmuth and 

 Springer find a difficulty in placing with the Oamerata. These two highly 

 specialised branches died out before the close of the Palaeozoic epoch, the Adunata 

 outliving the Camerata ; but the simpler Inadunate forms continued, and reached 

 a high degree of specialisation in their Jurassic descendants, to which the living 

 Hyocrinus is closely related. The Dicyclica Inadunata similarly gave ojff the 

 Dicyclica Camerata, which persisted only a little less long than their monocyclic 

 convergents. The dicjclic Crotalocrinidae of the Silurian are curiously parallel to 

 the Monocyclica Adunata, but it is not worth while to separate them from the 

 typical Inadunata. About the same time arose among the dicyclic Inadunata the 

 modification that resulted in the Flexibilia, with brachials loosely incorporated in 

 dorsal cup. The dicyclic Inadunata came to their acme in Carboniferous times : 

 their arms were then ' distinct,' and only those forms persisted to Neozoic and 

 Recent periods which assumed an Articulate modification — viz., a loose lateral 

 union of proximal brachials, as seen in Pentacrinidae, which are convergents of the 

 Neozoic Flexibilia. The latter Sub-Order was represented during Palaeozoic times 

 by the Ichthyocrinoidea (Impinnata). Between them and the Neozoic Apiocrinidae, 

 Bourgueticrinidae, &c. (Pinnata), the links are missing, but may yet be found 

 among Permian and Triassic crinoids {cide supra). At any rate, the Neozoic 

 Flexibilia, when they assumed the free-swimming habit, took a new lease of life 

 and have their acme in our own day. 



On the foregoing principles and beliefs, along with many others related in the 

 book itself, is based the following classification of Pelmatozoa : — 



SUB-PHYLUM PELMATOZOA.— Theca calcified and plated ; oral surface 

 uppermost, usually attached temporarily or permanently by aboral surface. Food 

 brought to mouth by ciliated grooves, which may be endothecal {i.e., between the 

 plates), epithecal, exothecal, or, in part and secondarily, hypothecal. Anus 

 usually in upper half of theca, never aboral. An aboral nerve-centre co-ordinates 

 the movements of the whole skeleton. Hydrocircus communicates indirectly 

 with exterior ; podia, when present, respiratory, not locomotor. 



Class I. CYSTIDEA. — Radial polymeric symmetry of theca developed either 

 not at all, or not in complete correlation with radial symmetry of ambulacral 

 and subvective systems. The latter is exothecal or epithecal, not endothecal. 



Order 1. Amphoeidea. — Radial symmetry has affected neither food-grooves 

 nor thecal plates, nor, probably, nerves, ambulacral vessels, nor gonads. 



Fam. Aristocystidce. No extension of food-grooves on theca or skeletal pro- 

 cesses; thecal plates irregular, unspecialised ; no stem. Dendrocystida. Single oral 

 skeletal process ; thecal plates irregular, merging gradually into stem. Anomalo- 

 cystida. Theca compressed in plane of thecal apertures ; the plates of the two 

 sides inclosed by a common frame of marginals ; food-grooves conveyed on spines, 

 one at each upper angle of theca ; no pores. 



Order 2. Rhombifeea. — Radial symmetry affects food-grooves, and (in more 

 advanced families) thecal plates ; probably also nerves and ambulacral vessels, but 

 not gonads. Food-grooves exothecal, the brachioles being either close to mouth, 

 or removed from it on a series of subambulacrals not derived immediately from 

 thecal plates, or separated from it by hypothecal passages. Stereom and stroma 

 become arranged in folds and strands at right angles to sutures. 



Fam. Echinospkaridee, Thecal plates numerous, indefinite, with pore-rhombs. 

 Brachioles circumoral, unbranched. Columnals not uniserial. Comarocystidce. 



