

A MONOGRAPH 



ON THE 



FOSSIL ECHINODEKMATA 



OF THE 



OOLITIC FORMATIONS. 



THE ASTEROIDEA. 



The true star-fishes forming the order Asteroidea have in general a depressed 

 stelliform body, which sometimes assumes a polygonal or pentagonal figure. From the 

 central disc five or more hollow rays proceed, containing prolongations of the internal 

 organs (PI. I, fig. 1). The entire upper surface is covered with a coriaceous integument, 

 in which a series of calcareous pieces, often supporting spines, tubercles, and pedicellarise, 

 are developed (fig. 2, a). In the centre of the under surface is the mouth-opening, from 

 whence radiate to the extremities of the rays as many ambulacra as there are lobes ; in 

 these, the tubular retractile feet are arranged in two or four rows ; and the margins of the 

 rays in many genera are bordered by well-developed sphiigerous plates. (PI. I, fig. 2, d.) 



The skeleton of the Asteroidea is a very complicated framework. It is composed of 

 a great number of little bones or ossicula, articulated together in such a manner as to 

 combine strength with flexibility. The ossicles vary in form and number in different 

 parts of the skeleton ; they have a persistent arrangement in the various genera, so that 

 the ossicula of a star-fish afibrd us good evidence of the rank of its owner among the 

 radiata, as the bones of a reptile or a mammal do amongst the vertebrata. The 

 comparative anatomy of the skeleton of the Asteroidea has not yet been worked out in 



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