184 UPPER, OR FLINTY CHALK. 



of more than a hundred specimens ; but as the recent animal is un- 

 known, and the fossils never occur in a perfect state, it is very probable 

 that some of those characters which are here assumed as permanent 

 distinctions, may hereafter prove to be only accidental varieties of form. 



29. Marsupites Milleri. Tab. xvi. figs. 6, 7, 8, 9, 13, 14, 15. 



Gen. Char. Body orbicular, contained in a pelvis composed ofcrus- 

 taceous plates, having five articulated arms or tentaculae proceeding from 

 the margin : the opening of the pelvis covered by articulated ossiculse, in 

 the centre of which the mouth is placed. 



Spec. Char. Pelvis composed of sixteen convex, radiated, angular 

 plates ; the arms dichotomous, united to the margin by a corresponding 

 number of intermediate semilunar bones : the ossiculae covering the aper- 

 ture of the pelvis disposed in a proboscideal form. 



The fossil remains of this zoophyte have liitherto been found only in the 

 uriper chalk of Sussex, and Wiltshire, and hke most other crustaceous 

 bodies enclosed in that deposit, are transmuted into a spathose calcareous 



spar. 



But one species is known ; the following description will therefore 

 illustrate both the generic and specific characters : 



This fossil is generally of a suborbicular form, more or less distorted, 

 with the lower extremity closed and obtuse, and the upper, truncated 

 and open, being filled with chalk or flint. It is composed of 'numerous 

 thin ano-ular plates, that are not united as in the echinites, but are simply 

 held in apposition to each other, by the chalk in which they are imbedded. 

 The name of " cluster stones" given them by the workmen, not inaptly 

 expresses their general appearance. 



The pelvis, or cavity in which the viscera of the animal were contained, 

 is very capacious, and is composed of sixteen angular, convex plates, 

 arranged in the following manner, viz. 



1 . A pentagonal plate {abdominal) placed in the centre of the base. 



2. Five pentagonal (costal) plates, attached to the sides of the centre. 



3. Five hexagonal {intercostal) plates, placed between the superior 

 angles formed by the union of the costal. 



