464 Royal Society. 



but very obliquely. The chambers, separated by these principal 

 or secondary septa, are long and very narrow, and extend from one 

 end of the body to the other. Their cavities are further divided 

 into chamberlets by tertiary ingrowths, which are generally at right 

 angles to the septa or nearly so, but are otherwise irregular in their 

 arrangement. No large primordial chamber, such as is common 

 among Foraminifera, has been yet discovered in Loftusin ; but its 

 absence cannot be certainly affirmed. In fully grown specimens the 

 turns of the spire, which succeed each other with tolerable regularity 

 at intervals of from l-iiOth to l-30th of an inch, are usually from 

 twelve to twenty in number ; but as many as twenty-five have been 

 counted in one instance, and a yet larger number might not impro- 

 bably be met with. The spiral lamina and its prolongations, form- 

 ing the accessory skeleton, are all constructed of almost impalpable 

 grains of sand, which is proved by analysis to have consisted of car- 

 bonate of lime, united by a cement of the same material. 



The Author then describes in detail the several components of the 

 fabric of Loftusia, and compares them with the corresponding parts 

 oi Parkeria. The continuity of increase of the spiral lamina always 

 leaves an open fissure between its last formed margin and the surface 

 of the previous whorl ; and through this aperture the whole system 

 of chambers included within its successive laminae communicates 

 with the exterior, through the passages between their cavities, which 

 are left in the building up of the septa. As already explained, the 

 labyrinthic structure takes its origin from the inner surface of the 

 impervious spiral lamina, the septa being directed towards the cen- 

 tral axis. These ingrowths have in many instances the form of 

 tubular columns, which traverse the chambers in a radial direction 

 (^. e. perpendicular to the spiral lamina), terminating either on the 

 septum of the previous chamber, or on the exterior wall of the 

 preceding whorl of chambers. But these tubes do not seem to be 

 homologous with the "radial tubes" of Parkeria, whose relations 

 differ in important particulars. The range of variation in a number 

 of specimens, as to the amount of the "secondary" and "tertiary" 

 ingrowths which divide and subdivide the chambers in Loftusia is 

 very great. The principal office fulfilled by this accessory skeleton 

 seems to be that of a support to the primary spiral lamina, im- 

 parting the necessary solidity to the organism. The degree of sub- 

 division of the chambers into chamberlets seems to have little bear- 

 ing on the general economy of the animal. 



The Author attempts to determine from the other Foraminifera, 

 of which the remains are found associated in the same Limestone 

 with those of Loftusia, what was its probable geological age, and 

 under what conditions it was deposited ; and he thence draws the 

 conclusion that the rock belongs to the lowest portion of the Tertiary 

 })eriod, presenting a niicrozoic fauna very similar to that of some of 

 our Miliolite Limestones, but richer in tlie small arenaceous R/iiso- 

 pods, and that the sea-bottom was a soft Calcareous mud lying at 

 a depth of from DO to 1 00 fathoms. 



