MORPHOLOGICAL PART. 



PRIMARY AND SUPPLEMENTARY PLATES. 



The plates of the Crinoids fall naturally into two categories, viz., primary 

 plates, and secondary or supplementary plates. The primary plates are the first 

 parts developed in the larva. They are represented in every group of the 

 class, and undergo comparatively few modifications in geological time. The 

 supplementary pieces appear in the growing Crinoid, but are unrepresented 

 in some of the groups. They are interposed between the primary plates, 

 and help to increase the capacity of the visceral cavity. They are very im- 

 portant in point of classification, offering by their presence or absence^ their 

 position and distribution in the calyx, their arrangement and multiplication, 

 excellent criteria for natural divisions. 



The primary plates may be subdivided into two classes : plates of the 

 abactinal system, and plates of the actinal system. The former are developed 

 on the right larval antimer, and include all plates connected with the cham- 

 bered organ and the axial cords. The latter are developed on the left anti- 

 mer, and communicate with the mouth and the annular vessels surrounding 

 it. The abactinal plates are represented by the stem joints, the basals, 

 infrabasals, radials, and the plates forming the dorsal parts of arms and 

 pinnules ; the actinal plates by the orals and ambulacral plates to the ends 

 of the brachial appendages. The remaining plates will be treated by us as 

 supplementary pieces. 



L THE PLATES OF THE ABACTINAL SYSTEM. 



A. The Stem and its Appendages. 



The length of the stem in some Mesozoic Crinoids must have been enor- 

 mous. Quenstedt traced that of a Jurassic Pentacrinus to 70 feet without 

 reaching either end. This is in striking contrast to its length in Palseozoic 

 forms. Among them the two longest stems observed by us have a length of 



