16 



LO WER IN VER TERRA TES. 



Fig. 12. — reiurojiolU 



ful Pohjstomella, and Penerojiolis, Fig. 12, common in all tropical seas. In the Ber- 

 muda sands the most frequently occurring genera are Penerojjolis, Fig. 1'2, Orbicu- 

 Una, and OrbitolUes, Fig. 15. These, with other light debris, are occasionally washed 

 out of the heavier matters of the shore by the action of the waves, and left in great 

 abundance in long, white streaks as the waves recede. 



Among the spiral shells there are two types, distin- 

 guished as nautiloid and turbinoid. TV^hen the spiral forms 

 in one plain, as in Polystomella, we have a nautiloid spiral ; 

 when it winds obliquely around a vertical axis, forming a 

 spiral like the snail or periwinkle, it is turbinoid. Tlie 

 beautiful Jiotalia, F'ig. 13, is formed upon the latter plan. 

 In most of the rotaline forms, all the chambers of the whorl 

 are visible from one side, but among the spii-als of the 

 nautiloid ty])e the later chambers often more or less en- 

 velop the older ones, so that unless one knows the struc- 

 ture of the shell it cannot be recognized by a cursory or superficial examination. 

 For example, in the very frequently occurring JVbniomna, the older chambers are 

 quite invisible, being entirely enveloped by the later ones, and in order to learn how 

 the shell began to form, a section would have to be made through it showing all the 

 chambers in one plane. 



Among the turbinoid spirals, there are several varieties of structure, the relations of 

 which are not easily seen until careful examination of the in- 

 ternal structure reveals them. Thus, Textidaria, Fig. 14, 

 belongs to the division, but at first glance it scarcely seems to 

 bear any relation to N'onionina. 



On close examination it will be seen that the successive 

 chambers are in two rows, and each chamber communicates 

 with the chamber above and the one below it on the other 

 row, never opening into a chamber of its o\vn row. 



In some species the nautiloid spire is characteristic only of the early period of 

 growth, for after a few turns, instead of budding from the end, thus continuing the 

 spiral, all the outer chambers put forth radial buds, which form successive concentric 

 rings. This mode of growth is well illustrated in OrbiloUtes, which is represented in 

 Fig. 15, part of the surface being removed to show the internal struc- 

 ture. It will be seen that the internal chambers are spirally arranged, 

 while the others are arranged on the cyclical or radial plan of growth. 



Dr. William B. Carpenter, whose ^-aluable monograph on the Fora- 

 minifera has thrown much light upon the structure and relationship of 

 these organisms, has shown the great importance of a careful study of 

 the shell-structure as a basis of classification. He has distinguished two 

 kinds of shell among the Foraminifera, which he has designated, re- 

 spectively, the porcellanous and the hyaline, or vitreous. These differ- 

 FiG. 14. — Tcx/u- ences of shell-structure corres])ond with physiological differences in the 

 organisms inhabiting the shells, and afford a basis for a division of the 

 class into two gi-eat sections. In both these sections will be found species which 

 have striking resemblances in form, which could not be generically separated except 

 by a recognition of the differences in the structui-e of the shell and their physiological 

 significance. 



Fig. 13. — Itolalia, enlarged. 



