OOLITIC ECHINODERMATA. 39/ 



which are limited in their range to the different stages of these great groups. 

 It is the object of. this paper to show that the species of the Oolitic Echino- 

 dermata had a limited range in time, and that the different stages of the 

 Oolitic formations are characterized by species which are special to each. 



Dr. William Smith was doubtless aware of the value of the Echinodermata 

 in stratigraphical geology, for he carefully noted the different species known 

 to him which characterized the secondary rocks ; and it is a remarkable 

 fact, that although our knowledge of the species of this class has been 

 nearly quadrupled since the publication of his works *, still the outlines 

 sketched by the hand of our great master remain nearly the same as laid out 

 by him. 



The test of the Echinodermata constitutes an internal and integral part of 

 the body of the animal, participating in its life, intimately connected with the 

 organs of digestion, respiration, and generation, as well as with those of vision 

 and locomotion, and having consequently many of the distinctive characters 

 of the organism indelibly impressed on portions of its skeleton. The individual 

 plates which compose the columns of the test of the Echinoidea, and the 

 ossicula which form the skeletons of the Asteroidea, Ophiuroidea, and 

 Crinoidea, are organized after distinct plans; they are therefore of great 

 value in determining the species, as the specific characters are often well pre- 

 served on even fragmentary portions of the skeleton ; for this reason the 

 remains of this class are of the highest value in stratigraphical geology, and 

 second to no other class of the animal kingdom in importance. 



In the Echinoidea the body is spheroidal, oval, depressed or discoidal, 

 and enclosed in a calcareous test or shell composed of ten columns of large 

 plates constituting the inter-ambulacral areas ; and ten columns of small plates 

 constituting the ambulacral areas, which segments are separated from each 

 other by ten rows of holes constituting the poriferous zones. '1 he external 

 surface of the plates is studded with tubercles of different sizes, in the dif- 

 ferent families; to these are articulated, by a kind of ball-and-socket joint, 

 the spines, which are of different sizes, forms, and dimensions in the different 

 families, and serve to characterize the genera and species. 



At the summit of the test is the apical disc, composed of five genital plates 

 perforated for the passage of the ovarial and seminal canals; and five ocular 

 plates notched or perforated for lodging the eyes : in one family, the Sale- 

 Ni adjE, an additional or suranal plate, composed of one or many pieces, is in- 

 troduced within the circle formed by the genital and ocular plates. 



There are two great apertures in the shell, one for the mouth, which is 

 alwa3's at the base ; the other for the anus, which occupies different positions 

 on the test; in one section it is in the centre of the upper surface, directly 

 opposite to the mouth, and surrounded by the genital and ocular plates ; in a 

 second section the vent is external to the circle of genital plates, and never 

 opposite to the mouth, but situated in different positions in relation to that 

 opening, being placed on the upper surface, on the sides, the border, the infra- 

 border or the base, in the different groups. 



The mouth is sometimes armed with a complicated apparatus of jaws and 

 teeth, but it is sometimes edentulous, or provided with lobes formed of the 

 plates of the test itself. 



The Asteroidea have a depressed stelliform body provided with five or 

 more lobes or hollow arms, which are a continuation of the body, and contain 

 prolongations of the viscera. The mouth is always below and central, and 

 rows of tubular retractile suckers occupy the centre of the rays. The com- 



* Strata identified by Organized Fossils, 4to, 1816. Stratigraphical System of Organized 

 Fossils, 4to, 1817. 



