278 FAMILY NUMMULINIDA. 



far more highly-developed organism, is met with in the seas of warmer latitudes, my largest 

 specimens of it being those collected by Mr. Jukes on the coast of Austraha. 



471. PoLYSTOMELLA CRISPA. External Characters. — The shell of T. crispa is usually 

 lenticular in form, its edge being sometimes rounded, and sometimes carinated ; its diameter 

 is usually from l-16th to l-25th of an inch. Not unfrequently, especially in young specimens, 

 short spines project at intervals from its margin. Only the last whorl of the spire is anywhere 

 visible ; this, however, does not extend as far as the centre,* the umbilical region being 

 occupied by a solid deposit of shell-substance, in which minute punctations can be distin- 

 guished. The septal bands, which are convex anteriorly, are very conspicuous, dividing the 

 surface into well-marked segments. On the exterior of each of these segmental divisions, 

 strong transverse crenulations present themselves, which are deepest near the convex margin 

 of the preceding septal band, where they terminate somewhat abruptly, and usually disappear 

 before reaching the concave margin of the subsequently-formed chamber. The depressions 

 between the elevated ridges often present the appearance of orifices ; but this appearance is 

 fallacious, since at no period in the growth of the shell is there any passage through these 

 depressions to the cavity of the chamber; the only communication which the system of 

 spirally-disposed chambers possesses with the exterior, being afforded by a variable number 

 of minute orifices which are to be found near the inner margin of the sagittate septal plane 

 (Plate XVI, fig. 5, c), close to its junction with the preceding convolution. These orifices, 

 however, are often discernible with difficulty from the exterior. When the surface of tlie 

 shell is examined with a sufficient magnifying power, it is observed to be crowded with 

 minute tubercles ; and this tuberculated structure is often especially evident upon the septal 

 plane, and in the rows of depressions between the segmental divisions. These tubercles often 

 so strongly resemble apertures, that it is only when thin sections of the shell are examined 

 under a variety of aspects, that their real nature is determinable with certainty ; and some 

 excellent observers (amongst others Prof. Schultze, xcvii) have been deceived into the 

 belief that they are pores. 



472. Internal Structure. — When the shell of P. crispa is laid open by a section passing 

 through the median plane, it is found to consist of a small number of convolutions, somewhat 

 rapidly increasing in breadth ; and each of these convolutions is shown by a vertical section 

 (fig. 5) almost entirely to enclose the preceding, the alar extensions of its segments being pro- 

 longed towards the centre, until they meet the solid umbilical nucleus. When the outer walls 

 of the chambers are examined from within (which is readily done by crushing a specimen, and 

 mounting the fragments in Canada balsam), a series of grooves are found to correspond with 

 the elevated ridges of the outer surface ; these grooves shallow towards the anterior or con- 

 cave margin of each segment, and deepen towards the posterior or convex margin ; and for a 

 short distance from the posterior septum each groove is converted into a tube by a narrow 

 lamella given off internally from the septum (Plate XVI, figs. 5, //, k'). These tubes, however, 

 establish no communication between the contiguous chambers ; for they are cnls cle sac, closed-in 

 by the lamella of the septum which formed the boundary of the previously-formed chamber. 

 In the living state they are occupied (as is shown by examination of the decalcified body, 

 Plate IV, fig. 28), by a set of processes h, b, of sarcode (see also Plate XVI, fig. 0, /?), which extend 



