830 MICROSCOPIC FORMS OF ANIMAL LIFE 



processes ' of the segments of the sarcode-body, the external walls of 

 the chambers being smooth. This form constitutes a transition to 

 the ordinary Nummuline type, of which Polysiomella is a more aber- 

 rant modification. 



The Nummuline type is most characteristically represented at the 

 present time by the genus Operculina, which is so intimately united 

 to the true Nummulite by intermediate forms that it is not easy to 

 separate the two, notwithstanding that their typical examples are 

 widely dissimilar. The former genus (fig. 628) is represented on our 

 own coast and in northern seas by very small and feeble forms, but 

 it attains a much higher development in the tropics, where its 

 diameter sometimes reaches one-fourth of an inch. The shell is a 

 flattened nautiloid spire, the breadth of whose earlier convolutions 

 increases in a regular progression, but of which the last convolution 

 (in full-grown specimens) usually flattens itself out like that of 

 Peneroplis, so as to be very much broader than the preceding. The 

 external walls of the chambers, arching over the .spaces between the 

 septa, are seen at 6, b ; and these are bounded at the outer edge of 



FIG. 629. Calcarina laid open to show its internal structure : a, chambered 

 portion ; 6, intermediate skeleton ; c, one of the radiating prolongations 

 proceeding from it, with extensions of the canal system. 



each convolution by a peculiar band, , termed the ' marginal cord.' 

 This cord, instead of being perforated by minute tubuli like those 

 which pass from the inner to the outer surface of the chamber- walls 

 without division or inosculation (fig. 632), is traversed by a system 

 of comparatively large inosculating passages seen in cross-section at 

 a', and these form part of the canal system to be presently de- 

 scribed. The principal cavities of the chambers are seen at c, c ; 

 while the ' alar prolongations ' of those cavities over the surface of 

 the preceding whorl are shown at c', c' '. The chambers are separated 

 by the septa d, d, d, formed of two laminae of shell, one belonging 

 to each chamber, and having spaces between them in which lie the 

 ' interseptal canals,' whose general distribution is seen in the septa 

 marked e, e, and whose smaller branches are seen irregularly divided 

 in the septa d', d', whilst in the septum d" one of the principal 

 trunks is laid open through its whole length. At the approach Of 

 each septum to the marginal cord of the preceding is seen the 

 narrow fissure which constitutes the principal aperture of commimi- 



