on the Organic Composition of Chalk and Chalk Marl. 383 



to that which is frequent in Madrepora and Astrsea, in which 

 the soft body is not divided or sharply cut off by com- 

 pact calcareous plates, but the soft parts appear interwoven 

 with minute calcareous rods, in a lattice like manner. These 

 numerous small connecting openings, which are sometimes 

 visible in some of the Rotalia and Rosalina, and also in the 

 Textularia, I do not consider essential, but hold that the true 

 channel of connexion has always a large diameter, and is sim- 

 ple for each single animal. The erroneous view of d'Orbigny 

 and of all his followers becomes so complicated, that polypa- 

 ries are held to be single animals, and consequently the vari- 

 ous connecting openings to be those of a simple individual. 



With respect to d'Orbigny's genus Nummidina, although 

 it has derived advantage from his diligent investigations, I 

 consider it as composed of very heterogeneous elements, which 

 belong to quite different divisions of animals. Some species 

 of the sub-genus Assilina, and perhaps all of them, may be- 

 long to the families Soritina and Asterodiscina^ while the As^ 

 silina nitida of the Red Sea is assuredly ForskaPs Nautilus 

 Oi'biculus, that is. Sorites Orbicidus, 



I am of opinion that all those species which are provided 

 with visible traces of mouths or openings, as in Lamarck's 

 genus Lenticulina with d'Orbigny's character of Nummulina, 

 are to be received among the Polythalamia; but that all such 

 species as have the form of a lens or disc, and are provided 

 with internal spiral cells, but without a trace of such mouths, 

 the cells being moreover separated from the external surface 

 by thick calcareous layers, are to be considered as internal 

 bones. These mouthless Nummulina are rather to be ranged 

 with the Velellida of the Acalepha along with Porpita, where 

 similar internally cellular coin-shaped bones exist. The con- 

 siderable size of many Nummulina is also striking and foreign 

 to Polythalamia, but agrees very well with the family of the 

 Velellida, as also in the want of traces of the attachment of 

 muscles, and in the want of a sipho or channel of connexion 

 between the cells. Until better informed, therefore, I de- 

 cidedly exclude the mouthless Nummulina from the Polytha- 

 lamia, and retain only Lamarck's Lenticulina in the sense at- 

 tached to d'Orbigny's Nummulina in a young state. 



The distinctive character of the Polythalamia, when com- 

 pared with their nearest relatives the Fhistra, Eschara, Cri- 

 statella, &c., consists in the shell, and in their freedom of mo- 

 tion. But with this may be combined the power of attaching 

 itself to other bodies, just as in the Cristatella (or Hydra also) 

 which often remains long attached, and then creeps again. 

 Those bodies which are apparently Polythalamian, but are 

 really adherent and immoveable, belong to the Cellepora, 



