.22 FORAMINIFERA OF THE CRAG. 



the cells differ in form and arrangement from those of OrbitolUes comjilanata \ but the 

 general form of the shell suggested Orhiiolites for its genus. OrbicuUna comjjressa is 

 indeed an isomorph of the well-known 0. comjj^anata (more properly 0. orhiculus) ; and 

 in some instances they are with difficulty separated. 



In distribution this variety is associated with the typical species ; wherever the latter 

 occiu's abundantly we see the tendency in the bolder specimens to take the characters 

 assigned to 0. compressa. 



In the spiral form of OrbicuUna we recognise the type of a series of large-sized discoid 

 Foraminifera common in tropical seas. The great diversity in appearance presented by 

 different mature specimens, and the alterations which take place from time to time in the 

 mode of growth of the shell, caused considerable confusion amongst the earlier writers, 

 and were the cause of much unnecessary division into " species." D'Orbigny, in his 

 ' Tableau Methodique/ and subsequently in the ' Cuba' Monograph, somewhat simplified 

 the nomenclature, by uniting the species founded by Lamarck, De Montfort, and others, 

 which were, in some cases, nothing else than the young, middle-aged, and adult of 

 the same variety ; but it was not until Professor Williamson, in 1851, published his 

 researches on the minute structure of the shell that the correct relations of the forms was 

 understood. No true specific difference exists between the specimens whose entire 

 growth is on a flat spiral plan and those which ultimately assume a discoidal form by the 

 alteration, after partial development, to a cyclical mode of increase ; neither has any 

 principle been found to account for this taking place. Another very variable character in 

 the species is the condition of the chambers in regard to subdivision. In well-formed 

 individuals each chamber is divided into chamberlets by transverse partitions ; but we 

 frequently find, especially in small or poor specimens, the chambers simple, and free from 

 any partition or constriction. The surface of the shell normally exhibits a certain amoimt 

 of surface-marking in the form of delicate parallel riblets, running in a transverse or 

 oblique direction to the chambers, very similar to those of Penerojilis ; but this, again, is by 

 no means a constant character. 



Genus — Orbitolites, Lamarck. 



Nautilus, Forskul. 



Orbitolites et Orbulites (parte), Lamarck. 



DlSCOLITHUS, Fortis. 



DiscoLiTES, jSIontfiirt. 



Marginopora, Quoy mid Gaimard. 



Sorites et Amphisorus, Ehrenberg. 



Orbitolites, Defiance, lyOrbiymj, Carpenter, Purler ami Junes, &c. 



