with Observations on their Internal Structure. 168 



posed of a very regular spire, embracing at all ages ; whorls often 

 very near together, not formed of many chambers, elongated 

 transversely, divided into a great number of capillary cavities by 

 partitions longitudinal to the whorl, the openings round, nume- 

 rous, and in lines transverse to the whorl*." 



CYCLOSTEGUES, D'Orbigny. 



" Animal composed of numerous segments placed in circular 

 lines. Shell discoidal, composed of concentric chambers, simple 

 or multiple ; no spire." 



"Cyclolina, B'Orb. 1839. Shell discoidal, each chamber 

 pierced by a number of pores making an entire circle round the 

 rest." 



" Orbitolites, Lamarck, 1801. (Orbulites, 1816, non Orbulites 

 cephalopodes.) Marginopora, Quoy et Gaimard, 1836. Shell 

 discoidal, plane, equal, and encrusted on both sides, presenting 

 concentric lines. Chambers numerous in irregular transverse 

 lines only visible at the border." 



" Orbitolina, D'Orb. These are Orbitolites with unequal sides ; 

 the one convex, encrusted, presenting concentric lines ; the other 

 concave, not encrusted ; presenting numerous chambers, in ob- 

 lique lines upon the side at the circumference." 



" Orbitoides, D'Orb. Shell discoidal, convex on both sides, 

 formed of a single range of chambers, round the disk, very thickly 

 encrusted about the middle, and presenting either radiating lines 

 or granulationsf." 



To these characters I shall add the following observations be- 

 fore proceeding further : — 



First as regards the distinction between the genera Assilina 

 and Nummulina. This is said to consist chiefly in the spire not 

 being embracing in the former, and which appears to be the case 

 to the naked eye. But if we make a vertical section of Oper- 

 culina, which, from its extreme thinness, is still further removed 

 from Nummulina than Assilina, it will be seen, under a magni- 

 fying power, to be formed of several layers, which may be traced 

 from the centre to the circumference, showing, that as the turn 

 of the spire is progressing, the deposition of new material not 

 only takes place at the margin but on both sides of the shell 

 generally, in a line from the last chamber in process of deve- 

 lopment up to the central or first-formed one. If then this can 

 be seen in a shell so thin as that of Operculina, how much more 

 evident must it be under the same circumstances in Assilina ! 



* Foram. Foss. du Bassin Tert. de Vienne, par M. Alcide d'Orbigny. 

 t Cours element, de Pale'ontologie et de Geologie Stratigraphique, par 

 M, Alcide d'Orbigny. 



11* 



