242 Dr. T. Wright on the Cidarida? of the Oolites. 



structure observable in this class can be resolved into a modi- 

 fication of the bilateral symmetry seen in the higher groups of 

 the animal kingdom. The elements of the skeleton are arranged 

 on two sides of a median line. If we take for example the Spa- 

 tangus purpureus, we observe that the test is elongated in the 

 direction of the line which connects the mouth with the anus ; the 

 mouth being situated at the base and nearer the anterior border 

 of the test, whilst the anus occupies an elevated position on the 

 posterior border. Were we to make a transverse section of the 

 Spatangus, we should have an oral or anterior half, and an anal 

 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 Spatangoidse is therefore very evident. In the glo- 

 bular forms of Cidaridse, 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 

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

 two columns of plates ; the ambulacral and interambulacral arese 

 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 sphaera 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 Cidaridse. 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 

 arese, and the ocular plates the summit of the ambulacral arese ; 

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



