44 



STUDIES IN EVOLUTION 



33 



35 



cup-shaped skeleton; as in Eucyrtidium elegans^^ Podocyrtis 

 Schomhurghi^^ Tridictyopus oonious,^^ Cornutella hexagona,'^^ 

 etc. Many of the primary, or axial, spines in other sub- 

 orders probably originated according to I. In the Spumella- 



rian forms especially, the princi- 



M^^^ pal spines project from the prom- 

 J^^^^ inent portions; as in Trigonactura 

 ^^^^^ triacantha,"^ Hymenactura coper- 

 nici,^ RJiopalastrum triceros,^^ R. 

 Jiexaceros,'^^ etc. The existence of 

 similar non-spinose species shows 

 that the formation of spines is in- 

 dependent of the growth of the 

 normal prominences; as in Rho- 

 palastrum malleus,^^ R. hexago- 

 num,^^ etc. 



In the Foraminifera the con- 

 figuration of certain forms is such 

 that parts of the test are much 

 more prominent than others, and 

 these exposed situations the 

 spines are most frequently devel- 

 oped. Some of the triangular 

 showing spiniform projection of Textularise have spines at the two 



the fundus. X 200. (After Leidy.) , , , ^, , -i 



lateral angles on the oral side. 

 Some of the individuals of Textu- 

 laria folium ^ show that similar 

 spines were developed at different 



36 



FiGDKE 32. — Atlheya decora, 

 a diatom, with spines from the in 

 angles. — (From Mic. Diet.) 



TiGUKE 33. — Dijflugia acumi- 

 nata, a, freshwater rhizopod ; 



FiGUKE 34. — DIfflugia con 

 stricta, a freshwater rhizopod, 

 with rounded fundus. X 175. 

 (After Leidy.) 



FiGDRE 35. — The same; 



showing a single spine on the g^^ges of growth, SO that, in a full- 

 fundus. X 175. (After Leidy.) ^ ^ . ' ' , 



Figure 36. The same ; show- grown Specimen, there may be two 

 ing two spines. X 175. (After or three pairs of spines along the 

 ^' ^ sides. Others, like Verneuilina 



spinulosa^ and Colivina pygmoea,^ develop spines from the 

 points of each chamber. A number of species, also, show a 

 single spine at the apex of the shell ; as Pleurostomella alter- 

 nans,^ Bolivina rohusta,^ Polymorphina sororia, var. cuspi- 

 data,^ etc. In the latter species the ordinary form is rounded 

 or obtusely pointed at the fundus. 



