Dr. T. Wright on the Cidarida? of the Oolites. 13"> 



or posterior half; whilst, on the contrary, were we to split the test 

 asunder in the line of its long diameter, we should have the right 

 half and the left half of the body. The five ambulacral area? are 

 unequal. The anterior area is not identical with either of the 

 others ; the first pair are symmetrical, but differ from the second 

 pair, which are likewise symmetrical; the bilateral symmetry of 

 these oblong Spatangoida3 is therefore very evident. In the glo- 

 bular forms of Cidarida?, however, a more careful study is requi- 

 site to make the demonstration complete. In them the test is 

 formed of polygonal plates united together by sutures and di- 

 vided into ten segments, of which five are named ambulacral 

 area?, and five interambulacral area?, each area being formed of 

 two columns of plates ; the ambulacral and interambulacral area? 

 alternate with each other, and are separated by ten zones of small 

 plates perforated for the passage of tubular retractile organs con- 

 nected with locomotion and respiration, and forming the porife- 

 rous avenues. 



The test of Echinus sphara is composed of twenty distinct 

 zones of elementary parts, which are narrow at the summit, 

 from whence they divide in rays, and gradually increase in 

 width towards the circumference or equator, where they are 

 widest; they again contract as they approach the mouth, which 

 occupies the base. The symmetrical disposition of these ele- 

 mentary zones occasions the radiated form which characterizes 

 the Cidarida?. Besides the plates of the ambulacra, interam- 

 bulacra, and poriferous avenues, the summit of the test is fur- 

 nished with a circle of plates surrounding the anus, composed 

 of five larger plates in relation with the generative organs, and 

 called ovarial, and five smaller plates disposed between them, in 

 which are lodged the organs of vision, and called ocular ; each 

 of the ten plates is perforated with a small hole for giving pas- 

 sage to the genital ducts and for lodging the eyes. This anal 

 circle of plates is called the apical rosette or disc. 



The ovarial plates occupy the summit of the interambulacral 

 areae, and the ocular plates the summit of the ambulacnd area? ; 

 the ovarial plates arc not all of equal size or of the same structure ; 

 one is larger and more prominent than the others, presenting a 

 spongy porous surface, and called the madreporiform plate ; it 

 is placed opposite the ambulacra, which is the analogue of the an- 

 terior area in the Spatangus, and occupies therefore the posterior 

 border of the apical disc, affording thereby a key for ascertaining 

 the antero-posterior diameter of the body ; the other four ovarial 

 plates are disposed in pairs before the single madreporiform plate. 

 The polygonal plates of both area? are arranged in double ver- 

 tical rows, two columns of ambulacral plates alternating: with 

 t\\<> columns of interambulacral plates ; the plates of each pair 



