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



Interambulacral areae 32 plates in each column 32 X 2 X 5= 320 plates. 

 Ambulacral area; 80 do. do. 80x2x5= 800 do. 



Poriferous avenues 160 do. do. 160x2x5=1600 do. 



Apical disc 10 plates 10 do. 



Each interambulacral plate supports 10 tubercles 320 X 10=3200 tubercles. 



Each tubercle supports a moveable spine 3200 spines. 



Each ambulacral plate supports 2 tubercles ... 800 X 2 1600 tubercles. 



Each tubercle supports a moveable spine 1600 spines. 



There are 70 rows of holes in each avenue, 



and in each row these six holes are disposed 



in pairs obliquely 70x6x 10=4200 foramina. 



The mouth in the Cidaridse is situated at the centre of the 

 basal surface, and provided with five jaws, each armed with a 

 long tooth ; the jaws are united by ligaments and moved by 

 numerous muscles belonging to the voluntary class. 



According to Prof. Brunner, the analysis of the test of Echinus 

 lividus gave the following result as its chemical composition : — 



Carbonate of lime .... 96-27 



Sulphate of lime 1*53 



Carbonate of magnesia . . . 0*93 



9873 



The fracture of the test and the spines presents a peculiar 

 crystalline surface altogether unlike that of the external skeleton 

 of other Invertebrata, depending probably on the manner the 

 salts of lime and magnesia are deposited in the cells of the ani- 

 mal basement membrane. The external and internal surfaces of 

 the test are covered by organized membranes, which extend 

 through the sutures and invest the spines and pedicellarise, and 

 are the producers and the sheath of the test and its appendages. 



The mode by which the spheroidal test of an Urchin main- 

 tains its original form, whilst it increases in all directions, is 

 easily understood after what we have stated relative to its com- 

 position. The viscera of the animal are inclosed in this fra- 

 gile and inflexible globular crust, which is never shed like the 

 external skeleton of the Crustacea, but grows by a process which 

 has some analogy with the expansion of the skull in the verte- 

 brate classes. By the division and subdivision of the hollow globe 

 into a number of elements inclosed between two layers of mem- 

 brane, additions are made to the periphery of the plates, whereby 

 they are enlarged and increase in thickness in proportion to the 

 requirements of the animal, so that the form of the test is main- 

 tained and its expansion provided for at the same time : the dif- 

 ference between the test of a young and an old Urchin chiefly 

 consists in the number and size of the plates entering into the 

 composition of the same. The new plates are developed around 

 the oral and anal poles, but chiefly near the latter region, where 



