80 Dr. G. C. Wallich on the Process of 



ternal sarcode of the chamber on which the spinous processes 

 occur has no direct share in their production, from the circum- 

 stance of there being no aperture or pit at the interior point of 

 the calcareous chamber which corresponds to the base of the 

 process exteriorly. It may be stated, therefore, that the process 

 of spine-formation in the Foraminifera is brought about by the 

 secretion of calcareous matter upon an already existing calca- 

 reous surface, each successive layer being received into the 

 mould progressively made for it by the outward extension of its 

 investing chitosarc. Fig. 5 diagrammatically represents the 

 process, sp being the first and sp' the second stage of spine- 

 formation referred to. 



On these grounds, then, coupled with the fact that no Fora- 

 minifera secrete silex, although several genera build up their 

 shells by the addition of siliceous and other mineral particles 

 derived from extraneous sources, I base my opinion that the 

 spicules met with occasionally in Polytrema, Carpenteria, and 

 Dujardinia* do not constitute integral portions of these or- 

 ganisms, but have their origin in entozootic growth ; and 

 further, that the process of mineral deposit in the Sponges, as 

 compared with that prevalent in the Foraminifera, is absolutely 

 incompatible with the formation of true spicular growth. 



In the Polycystina the plan of mineral deposit is in every es- 

 sential respect identical with that observable in the Foraminifera. 

 That is to say, the siliceous portions (which do not constitute a 

 shell, but ought to be regarded as an internal skeleton or frame- 

 work) are formed by the addition of siliceous matter at right 

 angles to the principal line of growth, and in one direction only. 

 They are never formed around stolons, and consequently are not 

 tubular — the finer threads of silex being projected from point to 

 point of the reticulations, much after the same fashion that the 

 threads of melted glass are fashioned by the glass- worker into 

 miniature baskets, &c., — the spinous processes so largely deve- 

 loped in this family forming no exception to the rule, and it 

 being only in the earliest rudiment of the siliceous structure 

 that the silex is deposited in the shape of an extremely minute 

 composite but solid spicule within the sarcoblast. This identity 

 in the plan of deposit in the Foraminifera and Polycystina, not- 

 withstanding the difi^erence in the mineral material and the cha- 

 racter of the hard parts, therefore yields a reason in addition to 

 those derivable fnom the organization of the soft parts for the 

 view advanced by me regarding the ordinal unity of these two 

 families. 



* On similar grounds I consider the marginal cord in Operculina to be 

 an ordinary secondary growth, in no wise analogous to spicular develop- 

 ment. 



