150 OF THE SUB-ORDER PERFORATA. 



characters of the central types would have to be so greatly modified for the compi-ehension 

 of the peripheral, as to lose all its precision. And all that can be here attempted will be to 

 point out what seem to be the typical peculiarities of each group, and to trace the shading 

 off of these as wc pass in different directions from its central t)^pe towards the most aberrant 

 modifications which it may present. 



227. Starting in the first instance, as in the Imperforate series, from the laonollialioiious 

 forms, we find among these a diversity of shape scarcely inferior than that which we have 

 seen to exist between Gromia, Squainulina, Cormif^piru, and Troclianimiiw. For in Orbtdina we 

 have a perfect sphere, not only devoid of any projection whatever, but very commonly 

 destitute of any principal aperture ; in ■ OviditeSy an egg-shaped spheroid, with an aperture at 

 each end ; in Lagenu, a spheroidal body drawn out at one extremity into a projecting neck, at 

 the end of which the aperture is situated ; whilst in SpirlUhia the' chamber consists of a very 

 elongated tube, regularly coiled in a flat spiral. There is a yet greater difference, however, 

 in the texture of these shells, than in their external form ; for the substance of that of Lacjena 

 is very finely tubular, whilst that of all the others is coarsely porous ; and the surface of 

 Lagena is almost invariably rendered uneven by regular longitudinal costation, or by some other 

 kind of symmetrical ornamentation, of which we scarcely find any traces in the others. 

 Moreover Lugma is distinguished by the peculiarity of its aperture, which is very regularly 

 circular, and has an everted lip, whose inner surface is commonly marked by radiating 

 fissures. 



228. Now it is not difficult to find a series of generic forms which naturally build 

 themselves up (so to speak) on the basis of the fundamental type presented by Lagena. For 

 if a new lageniform chamber be added by gemmation to the extremity of its predecessor, and 

 this process be repeated indefinitely in a rectilineal direction, we have a Nodosaria, — a type 

 which is characterised, like Lagena, by the finely tubular structure of its shell, by its central 

 radiating aperture, and by its peculiar ornamentation ; whilst a similar gemmation, repeated 

 along a bent axis, will give rise to a series of forms, more or less curved, but possessing the 

 same essential characters, bringing us at last to the flattened and symmetrical spiral of 

 CristeUaria. On the other hand, an oblique alternating gemmation will give us the biserial 

 Polymorphina or the triserial Uvigerina ; both of which types are manifestly related to Lagc7w 

 in their essential features, notwithstanding their striking isomorphism with Textiduria and 

 Vahultna respectively. — Now of the three principal families under which we shall range the 

 members of the Perforated series, that to which we give the name of Lagenida is unques- 

 tionably the one which presents the nearest approximation to the Imperforate ; for we shall 

 find that, notwithstanding the elaborate condition of its shell, even its highest forms agree 

 with the latter, rather than with the more elevated types of the former, as to the mode in 

 which the segments are united each to the preceding (^ G2), and in the absence of any trace 

 whatever either of "intermediate skeleton" or of "canal-system" (^ 63). 



229. If, on the other hand, we take Orbtdina as our starting pomt, and look for its 

 nearest alliances in the perforated series, we -shall have no difficulty in fixing upon Globigcrina 

 as its connecting link with higher t}'pes : the polythalamous shell of that genus being formed 



