18 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 



Gallery X. many of the Polythalamia were supposed to belong to the 

 Cephalopoda, that Class of Mollusca which contains the 

 nautilus and ammonite with coiled and chambered shells so 

 like those of some Foraminifera, but on an enormously 

 larger scale (see p. 151, Fig. 82). A. D. d'Orbigny, however, 

 whose attention had been directed to the shells found by his 

 father in the sea-sand near Eochelle, and who had studied 

 similar sands and fossiliferous deposits from various parts of 

 the world, published a memoir in 1826, pointing out that 

 these microscopic forms differed from the true Cephalopoda 

 in having no tube or siphuncle passing through the chambers, 

 but simply one or many holes or foramina through the walls 

 or septa that separate one chamber from the next ; he there- 

 fore distinguished them as " Foraminiferes " (hole-bearers). 

 By observation of the living animal, Dujardin in 1835 

 discovered the more essential difference that, whereas the 

 body of a nautilus is an elaborate structure confined to the 

 last-formed chamber of the shell, the body of a chambered 

 Foraminifer fills every chamber and is of simple protoplasm, 



Table-case connected throughout. Thus the flinty casts of the chambers 

 of some Foraminifera found in the flint-meal of Chalk flints 

 represent the form of the original animal, without the 

 pseudopodia (Fig. 2 h). 



The diversity of form assumed by the shell may be 

 studied in the two series of plaster models shown on the top 



Wall-ease shelf of the Wall-case. One of these series was made by 

 A. D. d'Orbigny and issued to his subscribers, about 1825, 

 in four " Livraisons " or sets of twenty-five each. The label 

 sent therewith described them as " Models of microscopic 

 cephalopods, recent and fossil, representing one example 

 from each of the chief divisions of a new classification based 

 on the mode of growth of the shell. The diameter of these 

 models is from 40 to 200 times that of the original shells. . . . 

 The coloured models represent the fossil shells ; the white 

 models, the recent shells. The position and shape of the 

 siphuncles [the openings between the chambers] are indicated 

 by the marks or black spots." The models are labelled with 

 the names originally attached to them by d'Orbigny, and are 

 arranged in the numerical order in which he sent them out. 

 Unfortunately that order was one of pure convenience and 

 did not correspond with his classification ; it is, however, the 

 order followed in a subsequent description of the models and 

 revision of their names by W. K. Parker, T. Eupert Jones, 

 and H. B. Brady (Annals and Mag. Nat. Hist., July, 1865). 



