with Observations on their Internal Structure. 165 



In Lycophris dispansus the central plane is extremely, though 

 uniformly, thin throughout, and only one chamber deep. The 

 chambers commence in an imperfect spire, round a central sphe- 

 roidal or oval cell, not much larger than the chambers themselves 

 generally. Around this cell are a few chambers which have — 

 one a semilunar, and two or three the pear-shaped forms of the 

 chambers commencing the spire in the nautiloid forms of ¥ora- 

 minifera (compare fig. 26. PI. VII. with fig. 7. PI. IV. vol. x.) ; the 

 rest are more or less polygonal. From these chambers (about 

 seven in number), as many rows of others fly off from the centre 

 in whorls similar to the sparks of a rotatory fire- work, but these 

 rows soon diminish in breadth, and end more or less abruptly 

 upon the back of each other ; when another set rises from their 

 circumference, which takes a larger latitude ; and so on success- 

 ively, a series of whorls or wreaths follow upon the back of each 

 other, until the rows appear to form concentric circles, still every 

 here and there dipping inwards, or suddenly terminating on the 

 preceding ones, even to the circumference. This is the appear- 

 ance presented by the central plane ; but the real spire must be 

 traced across the rows in the position that it would be in Fora- 

 minifera wherein it is more perfectly developed, if it be traceable 

 at all. 



In Orbitoides Mantelli*, however, the central plane is very dif- 

 ferent; here it is not uniformly thin throughout, but thin in the 

 centre and thick at the circumference, from the cells being only 

 half the size in the former that they are in the latter ; they are 

 also all spheroidal, or elongated vertically, and not quadrangular. 

 When they are elongated vertically, this seems to depend on two 

 or more running into each other in this direction ; hence the 

 central plane, instead of being composed of only one layer of 

 quadrangular chambers as in O. Mantelli, is composed of a plu- 

 rality of layers of spheroidal ones ; this, together with the small- 

 ness of the central cells, their great similarity, and the whole 

 plane which they compose being more or less wavy, renders it 

 almost impossible in the section to detect the central cell itself, 

 or to determine whether the others are arranged around it in 

 concentric circles ; while it seems almost equally impossible to 

 trace them in circles towards the circumference, to determine this, 

 where their arrangement even is most distinct. 



Hence it would appear, that D'Orbigny is not warranted in 

 giving the distinguishing character of concentricity to the rows 

 of chambers in his order Cyclostigues, for in his three first ge- 

 nera, which are all alike in this respect, we have seen that it is 

 almost impossible to determine this ; and in the last genus, viz. 



* Quart. Journ. Geol. Soc. vol. vi. p. 30. 



