236 FAMILY GLOBIGERINIDA. 



daries are formed by solid shell-substance (fig. 19, a). In the arborescent forms oi Polytrema 

 it is generally to be noticed that the extremities of the branches are rugged, instead of being 

 smoothly rounded off; and when these are looked at with a sufficient magnifying power, they 

 are seen to present several openings of considerable size (fig. 19, IJ). Similar openings are 

 sometimes observable in the solid spheroidal forms. Occasionally, moreover, solid tubercles 

 resembling those of Tlnoporiis haculatus are seen on the surface of the spheroidal forms. It 

 is a fact not undeserving of notice that specimens of Po///'/-e/«a which have been kindly placed 

 in my hands by Mr. Denis Macdonald (late of H. M.S. "Herald"), were completely coated over 

 with a membranaceous Sponge, the spicules of which seemed to radiate from the extremities 

 of the branches. Of the parasitic characters of this sponge, I can entertain no doubt what- 

 ever ; and it is a fact of some importance with regard to the presumed spongeous characters 

 of the body of Carpenferia (■[ 31.3), that although (as will be presently seen) the openings at 

 the extremities of the branches of Polyfrcma communicate very freely with the chambered 

 interior, I have not been able to find the least trace of the penetration of the spongy parasite 

 into its substance. 



410. Infernal Stmcfitre. — When the interior of Polytrema is laid open by section 

 (Plate XIII, fig. 20), its structure is found to be essentially the same with that of Tinoporus, 

 but less regular, and with freer communications between the chambers. In the arborescent 

 forms it is generally to be observed that in the centre of each branch there is a cluster of 

 longitudinal canals (a, a), each of which seems to have been formed by the coalescence of 

 several segments, or rather to result from a want of segmental division ; and it is 

 by the termination of these canals at the extremities of the branches, that the openings are 

 formed which are commonly seen there. The canals communicate with each other 

 and with the surrounding chambers by large, circular orifices ; and similar orifices of 

 communication, though of smaller size, are observed between the chambers elsewhere ; 

 but sometimes there are spaces of considerable size which present scarcely a trace 

 of subdivision, so as to bear a considerable resemblance to the chambers of Carpenleria. Thus 

 the whole shelly texture has ordinarily a less solid character than that of Tinoporus, although 

 formed on a plan essentially the same ; but we occasionally find an aggregation of calcareous 

 substance in solid pillars {b, b) exactly resembling those which we have seen in T. bnculatus 

 and in PateUina Cooki. The presence of these appears simply due to an exuberance of calci- 

 fying material; and it is obvious, from their extreme variability of distribution, that no reliance 

 can be placed upon them as furnishing differential characters. I have not been able to ascertain 

 with certainty the mode in which the growth of Polytrema commences ; but from the indica- 

 tions presented by the youngest specimens in my possession, I have little doubt that its 

 original development is essentially " planorbuline ;" that is, that its early segments are arranged 

 in an outspread spire, attached by its flat side ; and that when its growth becomes " acervu- 

 line," the newly added segments tend to go on piling themselves upon its upper surface, 

 instead of extending the disk horizontally. 



'o 



411. Affinities. — No lesson can be more instructive to the systematist who aims to 

 arrange the Foraminifera upon truly natural principles, than to trace out the affinities of this 

 singular genus. At the first statement of the proposition that the zoophytoid Polytrema is 



