CHAPTER YIII. 



• , FAMILY LAGENIDA. 



232. A more marked example can scarcel)' be found of the contrast between the results 

 of the system of classification adopted by M. D'Orbigny^ and those of the method followed 

 in the present work, than is presented by their respective modes of dealing with the members 

 of the group here brought together under a designation indicative of their essential relation- 

 ship to that minute Monothalam, which (as we have just seen) is one of the simplest forms 

 of the "vitreous" series of Foraminifera. It may be freely admitted that no set of charac- 

 ters can be framed, which should equally apply to all these organisms, and should differen- 

 tiate them from all other Foraminifera ; but they will be found to be mutually connected by 

 a strong family likeness in the most essential characters, — those, namely, which are most 

 indicative of the physiological condition of the animal ; and whilst we find one or other of 

 these to be shaded off", so to speak, as we pass from each generic type to the next, the persistence 

 of other features no less characteristic leaves no reasonable ground for doubt. Throughout the 

 group we find the hyaline or vitreous shell-substance to be possessed of great hardness, and 

 to be perforated by tubuli of extreme fineness ; in young and feebly developed examples it 

 is very thin and of almost glassy transparency ; but in older specimens, if well developed, 

 it is overlaid by superficial deposit, which in general not only adds to its thickness, but 

 rises into exogenous growths, having the form either of tubercles, of prickles, or most com- 

 monly of linear <os7«, which almost invariably run in a longitudinal direction, that is, parallel 

 to the axis of the shell. These exogenous growths, not being perforated by tubuli, have an 

 aspect that difi^erentiatcs them from the general surface of the shell. The small circular 

 aperture is situated in the centre of the septal plane, which is more or less prominent, and 

 is sometimes prolonged into a tube ; and it usually has an everted lip, divided by radiating 

 fissures into denticulations. These characters are presented in their highest development 

 by Lagejta ; and we shall see (^ 239, 253) that so perfect a transition exists between that Mono- 

 thalam and the polythalamous Nodomria, as renders it impossible to doubt the close affinity 

 of these two forms. From the rectilineal Nodosaria, through the gently curved DentaJhin and 

 the more strongly curved Jlarz/inuliua, we are led, in a series so jjerfectly gradationnl as to 

 forbid lines of demarcation from being anywhere drawn across it, to the spiral CrhteUarin, 

 which may be regarded as the highest manifestation of the lagenoid type. From this central 



