FAMILY LAGENIDA 155 



stem we find a set of offsets more or less divergent, presenting modifications of the typical 

 characters on which generic distinctions have been founded, but which we cannot 

 recognise as of higher than varietal importance. 



233. Now it is to be observed that notwithstanding the large size and high development 

 attained by many Nodomiice and Cristdlaria, their plan of structure is so simple as to bear a 

 stronger resemblance to that of the porcellanous series, than to that of the higher types of the 

 vitreous. For each segment is simply joined on to the preceding, in such a manner that the poste- 

 rior wall of the last segment is formed by the anterior wall of the penultimate ; the septal plane 

 of each segment is only that portion of its ordinary wall which is destined to be included by the 

 next addition, and is not differentiated in structure from the rest; and the septa dividing the 

 successive chambers are consequently not double but single. Hence there is not in this type 

 any vestige of an "intermediate" skeleton, notwithstanding the extraordinary amount of " sup- 

 plemental" deposit which forms the exogenous growths from the surface of these shells ; and 

 we find no trace whatever of any canal-system. Notwithstanding, therefore, the strong 

 resemblance in plan of growth between Crisiellaria and certain members of the Opcrculinc 

 series, there is a diversity in the most important features of their structure, which is quite 

 sufficient to mark their essential distinctness. 



234. Throughout the series which begins with Nodosaria and ends with Crixlcllarin, we 

 trace the same characters of shell-substance, the same forms of surface-ornamentation, the 

 same position of the aperture (modified only in appearance by the curvature of the axis), and 

 the same tendency to radiation at its margin, — ^the eversion of the lip, however, being for the 

 most part wanting. In Orthoccrina, with the same uniserial arrangement of the chambers, 

 we have an absence of radiation at the margin of the aperture, its form and position remaining 

 normal ; whilst the disposition of the exogenous growths resembles that of the preceding. 

 In Pol'i/morjiliina, on the other hand, the affinity to the lagenoid type is chiefly indicated by 

 the central position of the aperture and by the marked radiation of its margin, as well as by 

 the occasional existence of a tubular neck ; the external ornamentation is so far obsolete as to 

 be no longer characteristic ; and the plan of growth is biserial. In TJvujcr'nia, whose plan of 

 growth is triserial, the radiation of the aperture is wanting ; but its lagenoid nature is 

 marked by its elongated neck and somewhat everted lip ; whilst the ornamentation of the 

 surface of the shell is highly characteristic. 



235. In the classification of M. D'Orbigny, the closelj'-allied genera of this family 

 are distributed under /b//r different Orders, as follows : — 



I. — MoNosTEGUES. Lagcua. 



II. — Stichostegues. Glandulina, Nodosarid, Orthocerina, Dcntalina, Frondicularia, 

 Linguhna, Rimulina, Vaginulina, Marginulina. 



III. — H^LicosTiiGUES. Cristellaria, Flabclllna, Robulina, Uvigerina. 



V. — ENALLOsxi^GUES. Dimorpliina, Polymorphina. 



