GEIMLS VERTIiBRALINA. 7:5 



tliat afforded Ijy the sarcodc-ljody of the animal itself, ov by the gelatinous investment 

 of tlie sea-weed to which it adheres. Its growth nearly always eommences either in a regular 

 spiral or in a Milioline modification of it, each turn of the spire being commonly formed 

 (as in Hdiii'rina, *] 112) by three chambers, or more rarely (as in the ordinary MUiolee, 

 ^ 1 04) by two ; and it was on certain forms which are at first strongly milioloid and 

 afterwards uniserial, that D'Orbigny founded his genus Arliculinn. The earlier whorls 

 of the spire are generally inclosed by those which succeed them ; but sometimes they are 

 merely surrounded by them, so that the whole of the spire remains apparent. The septa 

 are marked externally by depressed bands, which are not crossed by striations. After 

 making from two to four turns, tlic shell continues to grow in a straight line ; the succes- 

 sive chambers (usually to the uuml)er of five or six) being sometimes of extremely uniform 

 size, subcylindrical in shape, and disposed with great regidarity (fig. 22) ; whilst in other 

 cases they are progressively compressed, so that the shell becomes flatter and wider 

 (fig. 25) ; and they are sometimes disposed in a zigzag manner. In either of these cases, how- 

 ever, the type is very easily recognised by the form of the aperture, which is a simple fissure 

 with lips slightly everted, extending along the whole breadth of the septal plane ; and it is 

 further to be noticed that behind each septal plane there is a more or less well-marked con- 

 striction, after which the walls of the chambers open out again. These constrictions are some- 

 times marked in an unusual degree by the very sudden enlargement of the new chamber 

 formed beyond, as in the specimen represented in fig. 23. Sometimes, on the other hand, 

 the tendency to increase appears suddenly checked, the new chamber being considerably 

 smaller than that which preceded it (fig. 21), the dimensions of which are only again 

 attained after a progressive increase continued through several chambers. A similar teraporar)- 

 dwarfing has been already noticed in Cornnspira, and seems liable to occur in any type of 

 polythalamous Foraminifera. 



97. Inlernal Sfrucftcre. — On laying open the shell of Vcrtchralhia, its cavity is found to 

 correspond very closely with its extei-nal contour, the walls of its chambers being everywhere 

 of very uniform thinness. The septal divisions are disposed at much more regular intervals 

 than in Nuhccularia, but still have rather the character of constrictions formed by an inflection 

 of the walls, than of regular partitions ; and the wide aperture in each septal plane can scarcely 

 be difl'erentiated from that of Nuhecularia, except in the somewhat patulous character which 

 it derives from the slight eversion of its lips. 



98. Varieties. — The typical form just described is liable to a great modification in cither 

 of two principal directions. On the one hand the chambers may become nearly cylindrical 

 in form, and at the same time narrowed and elongated, so that the straight portion of the shell 

 is drawn out into the form of a rod (fig. 19), though still presenting its characteristic con- 

 traction and subsequent opening-out at each septum ; whilst the aperture from a narrow- 

 fissure becomes circular, or nearly so. This rectilineal elongation of the chambers may 

 extend even to the first-formed portion of the shell, the primordial chamber and its imme- 

 diate successors being uncoiled (as it were) into a series of long cylinders laid end to end in a 

 straight line, the striated and pitted surface of which, together with the occurrence of 

 intermediate modifications of form, mark their derivation from the ordinary type. On the other 



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