1885.] NATURAL SCIENCES OF PHILADELPHIA. 22? 



was defective in not recognizing the more comprehensive relations 

 which exist among these animals. His groups failed to express 

 the distinctions in plan of structure, which we have pointed out. 

 While we are satisfied that the necessities of classification 

 require the recognition of a large number of family groups, which 

 we have not hitherto sought to define, we are more than ever 

 convinced that the three great groups which we originally estab- 

 lished, are the only really reliable ones, for the reason that they 

 are founded upon well-defined plans of structure. 



The Plates of the Abactinal System. 



Dr. P. Herb. Carpenter in his Challenger Report, p. 1, describes 

 " the organization of a Crinoid to be broadly divisible into two 

 well-marked portions," to which he applies the general names 

 "ambulacral and antiambulacral." The ambulacral portion is 

 " the visceral mass or disk in which is situated the whole of the 

 digestive tube with both its terminal openings, and it contains the 

 central ends of the radial water-vessels and blood-vessels." The 

 antiambulacral portion " consists of the stem and its appendages, 

 the calyx, and the skeleton of the rays, arms and pinnules." The 

 two portions, he states, correspond on the whole to the actinal 

 and abactinal systems of Echinoderms generally, and were de- 

 veloped, respectively, around the left and right water-tube, or 

 what are generally called the left and right larval antimers. 

 The whole of the calyx and the arm skeleton are formed on the 

 right antimer ; the disk and the extensions of the peristome, and 

 the perisomic plates clothing its ventral surface, on the left 

 antimer. 



In all recent Crinoids, and so far as known, in all Neocrinoids, 

 the calyx is restricted to the dorsal side of the Crinoid, and all 

 structures along the ventral side form a part of the disk or its 

 extensions. The calyx consists of few plates, as a general rule 

 only of basals and radials. Comparatively few genera have under- 

 basals. Interradials have been described only in Guettardicrinus, 

 in a few species of Apiocrinus, in Uintacrinus, and in the remark- 

 able recent genus Thaumatocrinus which exceptionally also has 

 anal plates. None of these plates, however, extend beyond the 

 limits of the dorsal cup. 



In the Palseocrinoidea the structure of the calyx is much more 

 complex. Underbasals are represented in nearly one-half of the 



