1885.] NATURAL SCIENCES OF PHILADELPHIA. 281 



Neocrinoidea, but existing among them as a constant character 

 through all stages of growth. In the Neocrinoid larva the orals 

 are large, occupying the entire ventral side, or one-half of the 

 entire test, and the conditions are not changed by the introduc- 

 tion of perisome in the adult. In the adult Palaeocrinoid the 

 actinal system of plates is restricted to a small space, and it is 

 very impi-obable that the orals extended out to the radials in the 

 earlier stages, or in such forms as Haplocrinus and Allagecrinus, 

 which, as admitted by Carpenter, are in the condition of the 

 Palaeocrinoid larva. 



All this tends to prove that the resemblance between the proxi- 

 mals and the orals in the adult Rhizocrinite or Thaumatocrinite, 

 and the " Scheitelplatten " in Allagecrinus and Haplocrinus with 

 the Pentacrinoid larva, is altogether superficial, and that the orals, 

 if these are developed in Palseocrinoidea, which we think they 

 are, can only be represented by the central plate. 



A resorption of the summit plates may have taken place in the 



later Inadunata ; throughout the Camarata they persisted through 



life. 



B. The Ventral Perisome. 



The ventral perisome covers the visceral mass or bod}- , and 

 together with the oral plates, forms the surface of the disk. It 

 is composed of the " ambulacral " and " anambulacral " plates. 

 The anambulacral plates are irregular pieces or limestone par- 

 ticles along the interpalmar areas, which consist of the anambu- 

 lacral plates proper, and the so-called interradial plates of the 

 disk. The former are pierced by numerous water pores, and 

 occupy the spaces between the ambulacra ; the latter are not 

 perforated, and occur in the substance of the perisome, uniting 

 the rays and their subdivisions. These interradials must not 

 be confounded with the calyx interradials, which cover those 

 of the disk. The ambulacral plates extend from the peristomial 

 area to the extremities of the. arms and pinnules, and consist 

 of the so-called " Saumplattchen " or " covering plates,'' and the 

 " adambulacral " or "side pieces" which support the former 

 and border the outer margins of the ambulacra. 



In the Pentacrinidse, the perisome is always studded more or 

 less with plates, and these often have a very solid appearance ; 

 while in the Comatulae the entire perisome is sometimes almost, 

 or totally, free from calcareous incrustations. 



