GENUS CALCARINA. 



217 



Fig. XXXIII. 



alike in number, length, and direction. These spines are usually few in young specimens, and 

 become numerous with age ; but this rule is by no means constant, since full-grown 

 specimens are occasionally met with having no more than four or five very short spines 

 (Fig. XXXV, h). The spines are usually either cylindrical or somewhat club-shaped, the latter 

 form being the more common ; but we occasionally see them showing a tendency to bifurcation 

 or trifurcation at their extremities ; and they not unfrequently appear as if two, three, or even 

 four spines had coalesced to form one, — this being indi- 

 cated not only by its unusual size, but by the mutual di- 

 vergence of its components as they extend themselves 

 from the central disk (Fig. XXXIII, b, and Fig. XXXIV, 

 f). a somewhat remarkable contrast in the relative 

 development of the disk and of the spines is presented 

 by the general aggregate of the specimens obtained 

 from the Philippines and the Mediterranean respectively ; 

 as is shown in comparing Figs. XXXIV and XXXV. 

 It is in the former that we meet with the greatest num- 

 ber as well as the greatest relative length of the spines ; 

 and that the bifurcation or trifurcation of their extre- 

 mities presents itself. In full-grown specimens of the Philippine Cdcarina, we com- 

 monly find the spines diverging from the margin of the central disk in such abundance. 

 Fig. XXXIV. Fir. XXXV. 



Two specimens of Philippine variety of Cul- 

 carina, distinguished bj unusual exuberance of 

 spinous outgrowths. 



Outline-representations of various specimens of 

 Philippine variety of Calcarina. 



Outline-representations of various specimens of 

 Mediterranean variety of Calcarina. 



that very little of that margin is left free. The length of their spines, moreover, at 

 different ages, varies pretty constantly with the diameter of the disk, the average pro- 



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