STRUCTURE OF POLYSTOMELLA. 507 



The peculiar surface-marking shown in the figure consists in a 

 strongly marked ridge-and-furrow plication of the Shelly wall of 

 each segment along its posterior margin ; the furrows being some- 

 times so deep as to resemble fissures opening into the cavity of the 

 chamber beneath. No such openings, however, exist ; the only 

 communication which the Sarcode-body of any segment has with 

 the exterior being either through the fine tubuli of its shelly walls, 

 or through the row of pores that are seen in front view along the 

 inner margin of the septal plane, collectively representing a fissured 

 aperture divided by minute bridges of shell. The meaning of the 

 plication of the shelly wall comes to be understood when we examine 

 the conformation of the segments of the sarcode body, which may 

 be seen in the common Pohjstomella crispa by dissolving-away the 

 shell of fresh specimens by the action of dilute acid, but which may 

 be better studied in such internal casts (Fig. 256) of the Sarcode- 

 body and Canal-system of the large P. craticulata of the Australian 

 coast, as may sometimes be obtained by the same means from 

 dead shells which have undei'gone infiltration with Ferruginous 

 Silicates. * Here we see that the Segments of the Sarcode-body are 

 smooth along their anterior edge h, b\ but that along their poste- 

 rior edge, a, they are prolonged backwards into a set of ' retral 

 processes ; ' and these processes lie under the ridges of the shell, < 

 whilst the shelly wall dips down into the spaces between them, so 

 as to form the furrows seen on the surface. The connections of the 

 segments by Stolons c, c 1 , passing through the pores at the inner 

 margin of each septum, are also admirably displayed in such 

 ' casts. ' But what they serve most beautifully to demonstrate is 

 the Canal-system, of which the distribution is here most remark- 

 ably complete and symmetrical. At d, d l , d 2 , are seen three 

 turns of a Spiral canal which passes along one end of all the seg- 

 ments of the like number of convolutions, whilst a corresponding 

 canal is found on the side which in the figure is undermost ; these 

 two spires are connected by a set of Meridional canals, e, e\ e 2 , 

 which pass down between the two layers of the septa that divide 

 the segments ; whilst from each of these there passes-off towards 

 the surface a set of pairs of Diverging Branches, /, f 1 , f 2 , which 

 open upon the surface along the two sides of each septal band, 



* It was by Prof. Ehrenberg that the existence of such ' Casts ' in the 

 Green Sands of various Geological periods (from the Silurian to the 

 Tertiary) was first pointed out, in his Memoir 'Ueber der Grunsand und 

 seine Einlauterung des organischen Lebens,' in " Abhandlungen der 

 Konigl. Akad. der Wissenschaften," Berlin, 1855. It was soon afterwards 

 shown by the late Prof. Bailey ("Quart. Journ. of Microsc. Science," Vol. v. 

 1857. p. 83) that the like infiltration occasionally takes place in recent 

 Foraminif era, enabling similar ' Casts ' to be obtained from them by the 

 solution of their Shells in dilute acid. And, acting upon this hint, Messrs. 

 Parker and Rupert Jones succeeded in obtaining from what had been 

 put aside as the refuse of Mr. Jukes's Australian dredgings, a number of 

 Casts of Polystomella, Alveolina, Amphistegina, and other types, of most 

 wonderful completeness. 



