100 FAMILY MILIOLIDA. 



which is perforated by multiple rows of rounded pores, each surrounded by a prominent 

 annulus of shell, and bearing a very close resemblance to the pores at the margin of OrbicnJina 

 and Orbifolites. One such row is pretty uniformly to be made out at the inner border of each 

 apertural plane ; and another row of very small pores, closely set together, may be detected 

 in well-preserved specimens along the outer border. The apertural surface increases con- 

 siderably in breadth towards its two extremities, and the rows of pores are there more 

 numerous but less regular. 



148. Internal Structure. — The internal structure of AJveoVma will be best understood by 

 supposing the cavity of an Oliva (or any other univalve shell that winds spirally round an 

 elongated axis) to be crossed not only by transverse partitions, which divide it into principal 

 chambers (indicated on the surface by the depressed septal bands that run parallel to the 

 axis), but also by a set of partitions at right angles to these, which divide each principal 

 chamber vertically into a row of long, narrow " chamberlets" in lateral contiguity with each 

 other, these secondary septa corresponding with the secondary furrows of the surface 

 (Fig. XXII). The chamberlets thus divided will open at the apertural plane by a single row 



Fig. XXII. 



Simple type of Alveoliim, as seen laid open at a, and in transverse section at B. At c is shown a portion of tlio apertural 

 plane of a specimen which exhibited two rows of pores ; and at d, a transverse section of one of the septa. 



of pores resembling that which is characteristic of Peneroplis. It may occasionally be noticed, 

 however, that the septal pores form a double row, as shown at c ; in which case those 

 nearest the e.Kterior are directed towards the longitudinal canal, as is seen in section at D, a, 

 so as to establish a direct communication between each segment and the longitudinal 

 stolon from which the next appears to commence. Such is, in fact, the structure of all 

 the fossil Alvoolina I have examined ; and we may characterise it as that of the si'wple 

 type of Aheolina. — In the recent form which I have specially studied, however, the structure 

 is much more complex ; for each principal chamber is subdivided not only by vertical, but 

 also by horizontal partitions or floors, so as to form a succession of " storeys," which are 

 indicated externally by the multiplication of the rows of pores on the apertural plane. This 

 will be partly understood from Plate VIII, fig. 15, which represents a section of Aheolina 

 taken in the direction of its elongated axis, and which shows the general mode of increase by 

 successive convolutions around the primitive spheroidal chamber, each convolution extend- 

 ing considerably in a longitudinal direction beyond its predecessor, and thus adding much 



