Mr.- H. J. Carter on the Polytremata. 193 



subconcentric or excentrlc foraminated layers upon each other, 

 in Polytrema^ as before stated. There is, however, no appear- 

 ance of " sand-grains " in this section of the structure of 

 AlveoUna meandrina^ which is so thin as to admit of being 

 examined by a 5-inch object-glass and transmitted light, when 

 even the tubulation of the spiral or foraminated layers is 

 visible. Of this species I stated (p. 328 I. c), it is " so 

 different from any other existing description, that at first sight 

 it seems doubtful whether it should not form the type of a 

 new genus." 



Another point which distinguishes Polytrema from almost 

 all the other chambered Foraminifera is the presence of the 

 aperture on the summit of the test, singly in the embryonic 

 form (PI. XIII. fig. 4, d) or in plurality in the full-grown in- 

 dividual (fig. 6, ccc). This may reasonably be assumed to 

 commence in the central cell of the disk which forms the base 

 of the embryonic test (fig. 4), and then only has its analogy 

 in Squamidina scoinda^ which appears to arise from the central 

 cell of the subraultilocular Rotaline test tliat forms its foot or 

 means of attachment to some submarine body ('Annals,' 1870, 

 vol. V. pi. iv. fig. 3, Ic). Of course the one-chambered or lage- 

 niform Foraminifera do not enter into this category. 



Still another point is the branching of the summit (PI. XIII. 

 fig. 1), which finds its analogue in Squamidina ramosa alone, 

 although in S. ramosa the branching, instead of stopping at 

 the first degree, may be continued on to the third ('Annals,' 

 1870, vol. vi. p. 347). 



Lastly, we come to the canal-system, of which there seems 

 to be an entire absence. Dr. Carpenter does not mention it ; 

 and I have not seen it. There is a fine polygonal linear net- 

 work to be seen in the centre of the unforaminated subhexagonal 

 reticulation with foraminated interstices which characterizes 

 the surface of an older form of Polytrema mimaceum ; but this 

 appears to be only a single straight linear canal, about l-12000th 

 inch in diameter (fig. 6, I), which originally communicated 

 with the hollow pillars now, in the older development, become 

 solidified. It is well represented in Max Schultze's figure of 

 an "Acervuline Planorbulina " in Dr. Carpenter's ' Introduc- 

 tion ' (pi. xiii. fig. 1), and might have been connected with 

 the original formation of the subhexagonal reticulation, which 

 does not appear in the newly developed structure (fig. G,fff) ', 

 and therefore, whatever this tubular network may be, it does 

 not come info existence until the former is developed. 



Directing our attention to the internal structure, we find that 

 the cancellated test is lined by a proper membrane (if a struc- 

 tureless sarcodic layer becoming brown and glue- or gum -like 



