FAMILY NUMMULINIDA. 239 



ment of the earlier whorls by the later is the typical plan, we not unfrequently find that llie 

 last convolution tends to detach itself from the spire, being adherent to its margin only, and 

 flattening itself out into so remarkable an extension as often completely to alter the original 

 contour of the shell. The aperture, in the typical nautiloid forms of tiiis family, is a narrow 

 fissure that is left between the outer margin of the penultimate convolution and the inner 

 margin of the septum ; and alike in its form, and in its position with regard to the two lateral 

 surfaces of the shell, it is consequently quite symmetrical. The fissure may, however, be 

 bridged over by processes of shell, so as to be converted into a row of discrete pores. 



415. The NuMMULiNiDA which conform to this general plan of symmetrical spiral 

 growth, may, therefore, in some respects, be considered as more nearly related to the higher 

 Lagenida than they are to the BotalincB ; for in respect to the nautiloid form of its shell, and 

 the fineness of its texture, there is an obvious resemblance between Cristellaria and Opercu- 

 liiia. But we shall find that this resemblance is superficial only ; the latter possessing a 

 complex internal structure which is wholly wanting in the former, but to which we have 

 found a very decided approach in the most elevated forms of the genus Botcdin. Now, it is 

 not a little remarkable that whilst llotulia thus forereaches on the Nummulinida, we should 

 find among Nummulinida a most distinct reversion to the Rotaline type ; the genus J//;yj///.s'- 

 ti'fjina having not only a unsymmetrical spire that sometimes approaches the turbinoid forui, 

 but having its aperture also on the under side of the spiral plane, and being also destitute 

 alike of the double septal laminae and of all trace of canal-system. In these last respects, 

 therefore, it is decidedly inferior to the highest forms of Rofalia ; yet in the arrangement of 

 its alar lobes and in its general morphology it conforms so closely to the Nummuline type, 

 that there can be no more question of its title to a place in this family, than there can be as 

 to the retention of the true Rofalia anionsr the Rotaline Globigerinida. 



's 



416. In the genus Polt/stomella, again, some of the characteristic features of the 

 Nummuline type are almost entirely absent ; the texture of the shell being far less elaborate, 

 the alar extensions in the later whorls being often deficient, and the spire being frequently 

 in some degree unsymmetrical. But we have, on the other hand, in this type, the highest 

 development of the canal-system that we anywhere meet with ; and even in those simple 

 " nonionine " forms which are among the most degraded examples of the Nummuline series. 

 we find this canal-system presenting a definiteness and symmetry much greater than that 

 which it exhibits in the most developed Rotalta. 



417. Passing onwards now from the typical NimmulincB to a higher plan of growth, 

 instead of reverting to a lower, we find that by a process of subdivision exactly analogous to 

 that by which a Pcneroplh becomes an Orblculina, the undivided chambers of Ojjcrculina are 

 converted into the rows of chamberlets, whose regular soiral increase is the essential cha- 

 racter of the genus Ileterostegina. If, however, this increase should take place on the cycli- 

 cal plan, as in Or/jitolifes, instead of on the spiral, we have a Ct/cIocIi/peHS, a type in which 

 the plan of structure of the individual segments is so completely conformable to that whiili 

 prevails in IIeicrostc(jhia, that marginal fragments of the two could no more be distinguished 

 from each other than could marginal fragments of Orhilolites and OrbicuUna. And, finally, 



