Cornish and Kendall—Calcareous Organisms. 73 
of the instability of Porcellanea is furnished by the list of Chalk 
Foraminifera in Dixon’s ‘‘ Geology of Sussex,” of which an analysis 
is subjoined. 
CuaLk ForaMINIFERA. 
EUGTLOLA ESD eRTALOW trasescteeadecsoetetcicescccasissceesisanosenes Porcellanea. 
SUILULOUCICM ANS DD eirsconicseacieeceeneerias cranes csieoseessiss ses Arenaced, 
LLG ERO, CRYESINS: sopodosd0a0 ¢ 356d00 008s a0aR EKO CoEaCObOEACUGCOONC Vitrea. 
Roliymonphinid@, 4 spp. AW VATS.. se. .ccce-enuececsesseseoree Ay 
JBN SOA UCE, MESS) Ns enogon cod coaGe Coe HOON COM DECCODBODECORCOEOE 70 
PRES UALONUA Ea OUSD Der seer cctceiseccnessnaceleseescecm sess. sacle 5 
GHODUGETINGPOVSPD Wosachraacsestoueandentlltore veer cared «dese 0 
TOU CIITA reepb a She orig de BOO ETOCS Cc ORC BOE RCOC LOS CCR OSE CCRRe nee aeEeE 5 
The significant fact appears from this that in a deposit from which 
all the aragonite shells have been removed, among 93 specimens of 
Foraminifera recorded, only one porcellanous form is included, and 
that presumably in so bad a state of preservation as to forbid specific 
determination. Judging from the analogy of recent Foraminiferal 
deposits from all depths, the Chalk should have contained originally 
a considerable proportion of porcellanous species. The chain of 
evidence was, however, less complete than in the case of larger 
structures, where casts nr replacements have been observed. We 
therefore tried the action of a solution of carbonic acid on Miliola 
(porcellanous) and Rotalia (vitreous) in the fossil state, with the 
result that the porcellanous species was reduced to a condition in 
which it was disintegrated by a stream of water, whilst the vitreous 
species maintained its integrity, neither did it lose its translucency 
or compact structure. It may be added that the strength of structure 
of the porcellanous is at least equal to that of the vitreous species. 
From experiment and observation we may therefore infer that 
Porcellanea are of inferior stability to Vitrea in presence of carbonated 
water, and consequently that their absence from a formation such as 
the Chalk does not warrant the assumption of their non-existence at 
the period of its deposition. 
Note.—Subsequently to the reading of the paper our attention was 
drawn to a communication by Prof. Sollas (Proc. Roy. Soc. Dublin, 
vol. iv. new series, 1885), in which the author comments on the 
rarity of Imperforate Foraminifera in the older rocks. The specific 
gravities were determined in the Sondstadt solution. The values 
obtained for recent specimens were Vitrea 2°626 to 2674, Porcellanea 
mostly 2°7, but ranging to 2°722. Wealso have succeeded in obtain- 
ing sufficient recent Spiroloculina for determination by the specific 
gravity bottle in a state of fine powder. Two experiments gave 
respectively 2°70 and 2°71. As we have not yet completed our 
examination of this group, and the percentage of animal matter has 
not yet been determined, we express no opinion regarding the 
mineralogical constitution of the tests, but content ourselves with 
pointing out that, as in the case of aragonite organisins, opacity in 
the fossil state accompanies instability towards carbonated water, 
both probably being related to the structure of the shell. 
