GENUS POLYTREMA. 235 



fossil forms are to be referred to these two types respectively ; and the diagnosis will probably 

 rest essentially on this, that whilst the surface-markings of the ujiper and under side are alike 

 in Tinoporus, though one side may be convex and the other concave, the convex side of Fatel- 

 lina always shows either a continuous spiral or a succession ofannuli of chamberlets. 



407. Geographical and Geological Disfribxtion. — The little PaiellincB which constitute the 

 existing representatives of this genus are very generally diffused, but do not seem to be any- 

 where very abundant. They have been found in Arctic, British, Mediterranean, Indian, and 

 Australian seas, between the littoral zone and depths of five hundred fathoms. — If we are right 

 in identifying with this type the Orbitolina lenticularis of Blumenbach and the Ci/clolina 

 crcfarea of D'Orbigny, this genus was highly developed at the Cretaceous epoch ; and the 

 same fundamental type seems to have presented itself under great diversities of form, 

 twelve or more different specific names having been given to its varieties. This genus is also 

 represented, as we have seen, by examples of considerable size in the Eocene Tertiaries of 

 India. Subsequently, however, it seems to have declined ; the Patellvia of the Grignon sands 

 being neither so large nor so well developed as those at present existing in the Australian 

 seas. 



Ge?mXXl.— PoLYTREMA (Plate XIII, figs. 18—20). 



408. History. — The genus Polytrema was instituted by Blainville (vi, «),for the reception 

 of the curious little bodies which had been previously described by Lamarck (lx), under the 

 name of Millepora rubra. Neither of these systematists appears to have entertained any 

 doubt of its Zoophytic character ; but its probable relationship to the Foraminifera was sug- 

 gested by Dujardin {xxxvi, p. 259), who, after speaking of the probably Rhizopodal nature 

 of Rofalia, PkuiorbitUna, &c., continues : — " J'ai bien constate que toutes les loges sont occu- 

 pees ^ la fois par la substance glutineuse ; mais je n'ai point vu les expansions filiformes, non 

 plus que dans le Polytrema rubra, que je conjecture appartenir a cette meme famille d'apr^s 

 la nature de la partie vivante." A similar view was afterwards taken by Dr. J. E. Gray 

 ('Proceedings of the Zoological Society,' April 27th, 1858), who further suggested that Polu- 

 trema might be specially related to Carpentcria, an idea which is not confirmed by examina- 

 tion of the internal structure of these types. 



409. External Characters. — Of all Foraminifera there is none so decidedly Zoophytic in 

 its form and Irnbit as Polytrema ; for although it sometimes spreads itself as a scaly incrusta- 

 tion on the surfaces of shells, corals, &c., it not unfrequently rises from those surfaces in an 

 arborescent form ; whilst sometimes its stalk, instead of branching, swells into a globular 

 protuberance, which bears a strong resemblance in size and general aspect to the globular 

 form oi Tiiioporim (Plate XIII, fig. 18). The surface is always areolated, but the character of 

 the areolation is not uniform, even on different parts of the same specimen ; for sometimes the 

 boundaries of the areolae are elevated as in Tinoporus, sometimes a little depressed as in Car- 

 pentcria, while sometimes they are on the same level with the spaces they enclose, and are 

 only distinguishable by their difference of texture, the areolae being porous, whilst their boun- 



