CABPlEXTEB ON Tin: AJIRAXGEMEKT of THE BHIZOPODA. 17L 



cement formed of the proper shell-substance, which is never wmtingj 

 and the closeaccordance in every other character between shells which 

 are thus Bliperficially altered and such us conform to the ordinary 

 type, forbids our regarding the former as more fhan varie tally distinct 

 from the latter. The case is very different, hotferi ''• with regard to 

 certain genera in which the power of forming a proper shell Beemsto 

 be altogether Wanting ; the testaceous envelope being essentially com- 

 posed of substance directly derived from without, the only material 

 furnished by the animal being the organic glue thai holds them 

 together, their substance is generally composed of* a wry fine cement 

 in Which coarser particles are imbedded; the former Sometimes predo- 

 minating, so that the shell is smoolhed off on the surface; whilst if 

 the latter be in excess, the surface of the shell is rough. Of this jam il y 

 the genus Liluola is the most characteristic ; and the variety offbnns 

 into which it passes, several of them so closely resembling those of 

 other genera as to have been mistaken for them, would not be readily 

 conceived by any but such as have made a special study of them.* 

 In the genus Trochammina (Parker and Eupert Jones) We have an 

 instance of a gradational transition from the monothalamous to the 

 polythalamous type ; for whilst its lowest form is a continuous ver- 

 micular spiral (the Spirillina arenacea of Prof. Williamson), this 

 comes to present, in some instances, a degree of segmental division 

 scarcely inferior to that which some of the most vermiculate forms of 

 Rotalia are reduced. The genus Valvulina forms the transition 

 between this group and the " perforated" series ; for whilst the prin- 

 cipal part of its " test" is uniformly made up of an aggregation of 

 sandy particles, leaving no such pores for the exit of pseudopodia, as 

 can be readily discerned in the arenaceous Textularice, this has a basis 

 of true shell- sub stance in which pores can be distinguished. 



In the whole of the sub-order Perforata, the shell is calcareous, 

 and is formed of a dense hyaline or vitreous substance, which is tra- 

 versed by tubuli running straight from the cavity of the chambers to 

 the external surface, whose diameter usually ranges from l-3000th 

 of an inch (as in Rotalia and Rlanorbulind) to less than l-10,000th 

 (as in Operculum and Cycloclypeus). There can be no question that 

 even the smallest of these tubuli are large enough to transmit the 

 finest threads into which the protoplasmic substance may sub-divide 

 itself: and looking to their remarkable continuity through suCces 

 layers of shell substance, when (as in Operculina) the earlier whorls 

 are completely embraced by the later, there can, I think, he no rea- 

 sonable doubt that, through their means, a fliredl communication is 

 maintained between even the earliest and innermost segments and 

 the surrounding mediiim.f This, of course, renders the successive 



* My knowledge of these arenaceous types has been entirely obtained through 

 the kindness of Messrs. Parker ami Rupert J< 



j The finely tubular shell-substance of Opefculind and its allies presents a 

 very striking resemblance to dentine in everything except the raniitication of the 



TOL. I. — H". If. If. BP 



