with Observations on their Internal Structure. 167 



specimens of the species I have met with respectively, and where 

 the characteristic structure externally has been too minute to be 

 seen by the naked eye, a small portion has been magnified in the 

 centre. Indeed in almost all, the lines and markings are larger 

 than they are naturally, and are therefore represented as seen 

 under a magnifying glass of low power, for in no other way 

 could these characters be given. 



As a typical description of Operculina and the structure of 

 foraminiferous shells generally, I must refer the reader to my 

 observations on O. Arabica, published in vol. x. No. 57 of this 

 Magazine, by a perusal of which an understanding of what fol- 

 lows will be much facilitated. 



Operculina, D'Orbigny. 



1. Operculina inaquilateralis (H. J. C). Inequilateral, oval 

 or discoidal, thin, horizontal or wavy ; centre prominent, margin 

 thickened, rounded, cord-like. Spire more or less irregular, 

 more apparent on one side than the other, consisting of three 

 whorls concave on one side, flat on the other, increasing rapidly 

 from a central cell. Chambers numerous, narrow, slightly re- 

 flected. Septa reflected, more apparent on one side than the 

 other. Diameter of largest specimens 5-24ths of an inch (Plate 

 VII. figs. 1, 2). 



Loc. Muskat in Arabia. 



Obs. This species difiers a little from D'Orbigny's characters 

 in being inequilateral, but the difi'erence between the two sides is 

 so slight, that it cannot be referred to any other genus. The in- 

 tercameral communication I have not been able to make out, and 

 although D'Orbigny almost invariably gives its shape and posi- 

 tion in the nautiloid Eoraminifera as a distinguishing character, 

 yet I have hardly ever been able to see it satisfactorily in any of 

 the species that I have examined. 



O. incequilateralis is a characteristic fossil of a thick, pink- 

 coloured, silico-calcareous, sandy stratum at Has Ghissa, the first 

 little cape south of Muskat, which is a port on the north-eastern 

 coast of Arabia opposite Scinde. I have inserted its description 

 here chiefly for the purpose of commencing with the simplest 

 form of nautiloid Foraminifera, and also from its proximity in 

 locality to Scinde. 



3. O. ? Equilateral, discoidal, plane or slightly wavy, 



thin. Centre prominent and presenting granulations or small 

 tubercles, projecting more in the young than in the adult state ; 

 tubercles situated over the septa, one to each ; margin slightly 

 thickened, rounded, cord-like. Spire more or less regular, equally 

 evident on both sides ; consisting of six whorls, gradually increa- 



