GENUS TROCHAMMINA. 141 



Genus I. — Trochammina (Plate XI, figs. 1 — 10). 



206. History. — ^The genus 7'roc^a?»wz«a was first instituted by Messrs. Parker and Rupert 

 Jones, in 1859 (lxxvii, p. 347), as a sub-genus of Botalia ; on the basis furnished by the 

 peculiar arenaceous form described and figured by Prof. Williamson (ox, p. 50, figs. 93, 

 94) under the designation Botalina inflata. The subsequent extension of their inquiries has 

 led them to separate it altogether from Botalia, and to include in it a large series of arenaceous 

 Foraminifera presenting very wide diversities both as to form and as to grade of development 

 (liv, p. 304) ; and it is on their conclusions that the following account of it is based. 



207. External Characters and Internal Structure.— Ihe, principal feature that is common 

 to all the forms of this apparently heterogeneous series, is the peculiar texture of the shell ; 

 which, although arenaceous, differs greatly from that of other genera whose shells are either 

 partially or wholly formed by an aggregation of particles of sand. The shell consists of a 

 dense ferruginous cement of an ochreous hue, obviously composed of an aggregation of very 

 finely divided particles, in which are imbedded sand-grains of somewhat larger size ; these 

 last do not project above the surface, which is finished-off like a smoothly plastered wall ; and 

 thus the shell of Trochammina contrasts strongly with the wholly-arenaceous shells of the other 

 genera of the same family, and with those partly arenaceous shells which sometimes present 

 themselves in other genera (as Nubecularia and Miliola), in all of which the sand roughens the 

 surface, its grains being larger, and the proportion they bear to the cement being greater. 

 The simplest form of Trochammina (fig. 2) consists of a cylindrical tube, gradually increasing in 

 diameter, attached by one of its surfaces, and spirally coiled, so as to bear a close resem- 

 blance in shape, on the one hand, to the cylindrical form of Cornuspira, and, on the other, to 

 SpirilUna. This form, already described and figured by D'Orbigny (xcii) as Operculina 

 incerta, and by Prof. Williamson (ex, p. 93, fig. 203) as SpiriUina arenacea, may be 

 distinguished as Trochammina incerta. Now the undivided spiral tube may form a vertical 

 spiral, instead of being complanate (fig. 3) ; and the resemblance it then presents to the 

 " nucule" of CJiara suggests the trivial designation charoides. In a third variety, T.gordialis, the 

 tube presents in its early convolutions an irregular division into chambers ; its later portion, 

 however, is undivided, and may either continue to coil in the original plane, or may raise 

 itself from this and pass over the earlier portion, forming loopings and twistings in various 

 directions (fig. 4), resembling those of a Gordius. Again, the subdivision into chambers may 

 become more complete, and the convolutions may assume a more regular disposition ; so that 

 the shell comes to present a series of lunate flattened chambers, several in a whorl, regularly 

 increasing with the progress of growth, and communicating by fissures corresponding to those 

 of Botalia in position, so as strongly to resemble in general conformation the flatter varieties 

 of B. turbo (fig. 1); this form may be distinguished as T. squamnta. A still higher 

 development on the same plan (fig. 5), gives the form already noticed as having been 

 described under the designation B. injlata, which may be regarded as the highest type 

 of this series. — But returning again to the original undivided cylindrical tube, we find 

 that this may undergo another series of modifications, not at all less remarkable, 



