PRINCIPLES OF CLASSIFICATION. 



49 



" nioiiollialamoiis " I'oramiuifera, the contraction of the aijerture entirely forbids the enlarge- 

 ment of the ca\ity by any sucli addition, but those spiral forms in which there is no such 

 contraction are capable, like t!ie tubular shells of Annelids and certain Mollusks, of bcin"- 

 indclinitely extended ; and it is simply because of its absence, that they present no indication 

 of the segmental division which would otherwise be marked by a transverse septum every time 

 that a longitudinal addition is made. This is the case with the genera Cormcsjjmi, Trochammina, 

 and SpirilUiiri, which arc respectively the porcellanous, arenaceous, and hyaline representa- 

 tives of the simply spiral plan of conformation ; and it is obvious that, although structurally 

 " monothalamous,'' these genera are in reality more nearly allied in their capability of 

 unlimited growth to the " polythalamous" series. The " polythalamous" shells are formed, as 

 already explained, by the repeated gemmation of the sarcode-body ; and it usually (though 

 by no means constantly) happens that the successive gennnaj increase in size, so that each 

 chamber is larger than the one which preceded it. The simplest types of Pohjthalntnki arc 

 those in which the segments of the shell have only an external adhesion, the cavities of their 

 chambers having no communication with each other ; as is the case in the im^jcrforatc series 

 with Daciz/Jopora and Aclcidaria, and in \\\q perforated with Glob'igerina. Save in these types, 

 however, the cavity of each segment always communicates with that of the segment from 

 which it is budded off ; and this it may do merely by the apertural neck of the older segment, 

 which dilates into the globular cavity of the newer, so that the succession of segments is 

 united into the semblance of a string of beads, as we see in some forms of Nodo>iaria (Plate 

 XII, fig. 2). More generally, however, the apertural portion of one segment is completely 

 embraced by the walls of the next chamber, in the manner diagrammatically represented in 

 Fig. Ill, where the primordial chamber is seen at and its aperture at n ; this aperture is 

 received into the second chamber, 1, the lateral 

 wall of which joins the anterior wall of the preced- 

 ing at h ; in like manner, the aperture «^ of the 

 second chamber is received into the third chamber 



2, the aperture of this, «°, into the next chamber 



3, and so on, the aperture n"^ of the last chamber 

 opening externally. — A great alteration in external 

 shape may be produced, without any departure from 

 the rectilineal plan of growth, by the still more com- 

 plete reception of the anterior portion of the older 

 chamber into the posterior portion of the newer; as 

 is shown in Fig. IV, wliich diagrammatically repre- 

 sents the " frondicularian" form of Nodosarina. — 

 On the other hand, a complete alteration in external configuration may be pioduced by the 

 substitution of a curviliiirar for a reclilliiear axis of growth ; the most ordinary type of the 

 former being a spiral, which may be either flat, like that of a Nm/tilns, or may coil around a 

 longitudinal axis, like that of a Trocluis. Between the rectilineal and the spiral forms of axes 

 of growth a complete gradation is presented in the Nodosarine series. In Fig. VII we see 

 that the relations of the chambers may be essentially the same in the spiral as in the recti- 

 lineal type ; the septal plane that divides each chamber from its successor being formed 

 solely by the anterior wall of tlie older, which serves as the posterior wall of the newer. 



Fig. III. 



Fig. IV. 



