284 FAMILY NU.AIMULINIDA. 



immediate neighbourhood of its junction with the spiral lamina constituting the external wall 

 of the chamber; and that they are thus homologous with the arches of the interseptal 

 system of canals that connect together the spiral canals of OpercnJina (■[ 445) — presenting, 

 however, a much greater uniformity and constancy in their disposition. The diverging 

 branches given off from these pass at once into the spiral lamina, beneath the ridges of the 

 shell which intervene between the furrows for the lodgment of the retral processes, as seen in 

 fig. 8 ; and through the thickness of the spiral lamina they run obliquely towards the external 

 surface of the convolution, usually increasing in diameter as they proceed. The divergence of 

 the branches of each meridional canal causes those proceeding from adjacent canals to 

 approach one another; and when the spiral lamina has attained its full development they 

 not unfrecjuently open at its surface into the same depression, this being midway between the 

 septa from which they respectively sprang ; and it appears to be from the correspondence of 

 these junctions with the intervals between the segments of the succeeding whorls, that the 

 alternating arrangement of the chambers of consecutive whorls arises, of which mention has 

 already been made (^ 475), the prolongations of sarcode which occupy the diverging branches 

 there passing into the stolons which connect the adjacent segments. It will be easily under- 

 stood, however, that the position of the external orifices of these diverging branches will 

 depend upon the thickness of the spiral lamina which they have to traverse before gaining its 

 surface. In the newest portion of a shell which has not yet attained its full growth, we find 

 that lamina comparatively thin ; its surface is distinctly marked by the septal bands (fig. 1, 

 <ig' (/(J) ; and the external walls of the chambers present an alternation of ridges and furrows 

 passing directly across from one septal band to another — the ridges corresponding to the 

 grooves of the internal surface that receive the " retral processes " (^ 476), and the furrows 

 with the internal ridges that separate these grooves. Into these furrows, wdiich represent the 

 deeper " fosscttes " of P. cri^jja, the diverging pairs of branches from each meridional canal open 

 by minute pores on either side of the septal band, as is shown aty/, yy', fig. 1. The subse- 

 quent formation of a calcai'eous deposit, continuous with that which solidifies the umbilical 

 portion of the shell, upon the external surface of the spiral lamina, renders the septal bands less 

 distinct, and obliterates the ridges and furrows of the intervening surface, as is shown in the 

 portion h //of fig. 1 ; and at the same time it carries the orifices of the diverging bi-anches from 

 the neighbourhood of the septa into closer proximity with those of the branches proceeding 

 from the adjacent meridional canals. As the diverging branches enlarge greatly in diameter 

 with their augmentation in length, their superficial orifices become more and more conspicuous ; 

 each is surrounded by a little pit or depression of its own (fig. 1, ii' , ii'); and the rows of 

 these depressions, when the spiral lamina has acquired its full thickness, constitute the only 

 markings wdiich it presents, the septal bands being completely obliterated, — as is best seen 

 on the surface of one of the interior whorls exposed by the removal of that which covered it. 

 The removal of the superficial portion of the spiral lamina, however, even when it is thickest 

 (which may easily be accomplished by the assistance of dilute acid), brings back these orifices 

 of the diverging canals to the immediate neighbourhood of the septal bands, which then 

 again become apparent. It is obvious, therefore, that these depressions in the thickened 

 portion of the shell of P. craticulata, being related only to the distribution of the canal system, 

 are essentially different in character and position from the superficial depressions of P. cr'ispa, 

 which intervene between the ridges that cover-in the retral processes. 



