GENUS ALVEOLINA. 103 



vertically along the septal plane in fig. 13, would establish what I have termed the "columns" 

 which seem to constitute the first-formed portion of a new segment ; a lateral coalescence of 

 these columns at tolerably regular intervals would establish the longitudinal " stolons ;" and the 

 formation of solid shell between these columns and stolons would leave them in occupation 

 of the two sets of communications that have been designated as the " well-staircases " and 

 the " horizontal galleries." The further development of the segment will consist in the 

 anterior extension from each " column" of a set of sub-segments of sarcode separated from each 

 other both vertically and horizontally by partitions of shell ; and the extremities of these will 

 appear, when the growth of the segment is completed, at the apertural pores, where they will 

 be again connected through the coalescence of their extensions, so as to form vertical 

 " columns" and longitudinal " stolons." 



150. Varieties. — The foregoing plan of structure may be traced, with or without 

 modification, through a series of extinct forms which presents a wide range of variation in 

 shape, and which closely approximates at one extremity to that of Orbiculina. Thus the 

 (so-called) Orbiculina rofella of D'Orbigny (lxxiii, p. 140), notwithstanding its nautiloid 

 discoidal form, seems pretty certainly to belong rather to the Alveoline than to the Orbiculine 

 type of structure ; from this the transition is by no means abrupt to the oblately spheroidal 

 and thence to the spherical varieties of Alveolina melo, which is a very common fossil of the 

 early Tertiary Limestones both of the Continent of Europe and of India. From its spherical 

 we pass to its prolately spheroidal forms, and thence to its ovoidal (the A. ovoidea of 

 D'Orbigny), some of which last attain enormous dimensions, specimens being not unfrequent 

 in the Tertiary Limestones of Scinde whose length reaches three inches and whose diameter is 

 an inch and a half. A still greater elongation of the axis, with attenuation towards the extremities, 

 gives the fusiform shape of the A. Boscii, a variety which is common in the Paris Tertiaries, 

 and which bears a strong external resemblance to the one at present existing in tropical seas ; 

 and it is clear that the yet more elongated sub-cylindrical forms presented by A. elongata and 

 A. Quoii, of which the former is fossil and the latter recent, are nothing else than a result of 

 the tendency to elongation of the axis carried out to a still greater extent. There is, however, 

 an important difference in internal structure between the entire series of fossil forms and most 

 of their existing representatives ; for although the external aspects are so nearly identical in 

 certain specimens that they are only distinguishable by the difference in the number of rows 

 of pores in their septal planes, and although the general plan of the fossil is exactly con- 

 formable to that of the recent, yet that plan seems to be uniformly worked out in fossil 

 forms, however large may be their dimensions, upon the " simple" plan, the " complex" 

 being only met with in the type at present inhabiting equatorial seas. Noyv this 

 difference, being apparently well marked and constant, would obviously be entitled to rank 

 as of specific value ; if we did not find in Orbitolites, to which Alveolina is closely allied in 

 general plan of structure, that a difference of an exactly parallel character between what will 

 be there termed the " simple " and the " complex " types must be reduced to the grade of a 

 mere varietal modification, since the passage from the former to the latter is frequently 

 presented in one and the same individual.* 



* It seems requisite for me to state, that I fiud myself unable to admit the existence of any 



